【Abstact】Second Langauge Acquisition is the common term used for the name of discipline. In general， SLA refers to the learning of another language after the native language has been learned. We can define second language acquisition as： no matter in class or outside class， people learn any other language except their mother tongue. The vocabulary acquisition is the important aspect of the SLA. Vocabulary learning is the most important part of English learning， only with the large amount of vocabulary can we fully understand the meaning that others want to express and can we express ourselves freely.
　　【Key Words】 Second language vocabulary acquisition； incidental acquisition； intentional acquisition； vocabulary teaching
　　【作者簡介】张瑞（1993.4.3-），女，汉族，甘肃陇南人，西北师范大学，研究生在读。
　　1.Introduction
　　Lexical competence is an important part of language communicative competence. For Chinese learners who studying English， the importance of vocabulary learning is very obvious， and it runs through the whole language learning process. A certain amount of vocabulary is the foundation of language activities such as listening， speaking， reading， writing and translating. In our daily life， when we talk to our friends and family and others， vocabulary is the most important.
　　2.The definition of vocabulary acquisition
　　Vocabulary acquisition is a basic task and key element in foreign language learning. It is closely related to the four learning skills： listening， speaking， reading and writing， and promoting each other. It should be said that second language learning is different from the learning of mother tongue. It requires higher initiative of learners in their own learning， such as imitating the pronunciation of words consciously， distinguishing synonyms， paying attention to the collocation of words， and so on. The task and purpose of vocabulary acquisition is to transform negative words into positive words as much as possible.
　　3.The characteristics of the second language vocabulary
　　English Lexicology experts generalized the characteristics of second language vocabulary with three adjectives： copious， heterogeneous， varied. Copious refers to the large number of English vocabulary. In 2013， the eighth edition of the Oxford dictionary published including 184500 words. This dictionary deleted a large number of words in the seventh edition that are difficult to understand or are rarely used in our daily life， but also added a lot of new words. Heterogeneous refers to the wide range of sources of English vocabulary. Many English lexicologists have pointed out that the reason why English absorbs such a large amount of foreign language， it is the result of external and internal factors. Varied refers to the large usage of of English vocabulary. One of the most prominent features of English vocabulary is that the synonyms are particularly rich， which is the result of tborrowing a large number of foreign languages. 　　4.Incidental vocabulary acquisition and intentional acquisitionin second language vocabulary acquisition
　　Incidental vocabulary acquisition refers to the unintentional acquisition of vocabulary when the learner accomplishes other learning tasks rather than memorizing the target words. Incidental acquisition is a psychological concept and is first proposed by Nagy Herman and Anderson. They found it in the process of the research of children’s acquisition of mother tongue， children speak English as their mother tongue can grasp 3000 words， but not all of these words are acquired from the classroom teaching， or probably just learned in daily life learning.
　　Intentional acquisition is sometimes called language learning， and learners focus their attention on the learning of linguistic features， such as pronunciation， spelling， vocabulary， grammar， and discourse out of context. The concept of intentional vocabulary acquisition is derived from this. Intentional acquisition is to learn with the purpose of mastering the pronunciation， spelling， meaning of words. Intentional vocabulary acquisition is the opposite to incidental vocabulary acquisition.
　　5. Conclusion
　　In a word， in vocabulary acquisition， first， we should organize a large number of extensive reading and listening activities to improve the consciousness， and the repetition of new words can promote the acquisition of vocabulary. Secondly， incidental acquisition and intentional acquisition should be combined. Teachers can also design some exercises related to words that learners pay more attention to these words， or teachers can guide students to guess the meaning of words， or look up the meanings of words in the dictionary. As long as the students persevere and accumulate over a long period， they can learn vocabulary easily.
　　References：
　　[1]Ellis， R. The Study of Second Lnaguage Acquisition[M]. Oxford ： Oxford University Press， 1994.