There is a long-term problem that the population size determination in urban master planning usually deviates from the implementation result.How to determine the population size scientifically and reasonably during the transformation of spatial planning system in China has become the focus of attention in the new era.Taking Xi\'an as an example,based on the analysis of differences between the population size determination and the actual development of population in the four rounds of urban master planning since the founding of the People\'s Republic of China in 1949,this paper reflects on the reasons for the inaccuracy of population estimates from three aspects:the connotation of“goal-oriented”planning,the planning objective of“growthism,”and the determination method of“mechanical simplicity.”In the context of territorial and spatial planning,this paper puts forward enlightenments for the population size determination in urban master planning in three aspects,i.e.,putting the people first and combining rigid and elastic control,practicing ecological civilization and strengthening bottom line thinking,and establishing a sound methodological system and unified standards and regulations.