Flower-bud differentiation is the most critical stage of fruit yield formation,as well as a complicated carbon transfer process. To understand the changes of carbon during the flower bud differentiation process of longan,it is very important to adopt suitable planting measures and control flowering time. This study focused on the changes of sugar and starch content of on-season and off-season longan during the flowering process. The results showed that the accumulation and transformation of carbon differed between on-season and off-season longan during flower-bud differentiation process; to be specific,sugar and starch content in tissues differentiated from apical buds of on-season longan were reduced at the late flower-bud physiological differentiation stage except in leaf buds; on the contrary,sugar and starch content in off-season longan increased at the late flower-bud physiological differentiation stage: specifically,total sugar content increased from15. 43(- 7 d) to 31. 38 mg /g( 7 d); starch content increased from 5. 42 to 8. 31 mg /g during this time,which may be relevant to the external environment or tree original metabolism damaged by potassium chlorate. To understand the changes of carbon during the flower bud differentiation process of longan, it is very important to adopt suitable planting measures and control flowering time. This study focused on the changes of sugar and starch content of on-season and off-season longan during the flowering process. The results showed that accumulation and transformation of carbon differed between on-season and off-season longan during flower- bud differentiation process; to be specific, sugar and starch content in tissues differentiated from apical buds of on-season longan were reduced at the late flower-bud physiological differentiation stage except in leaf buds; on the contrary, sugar and starch content in off- season longan increased at the late flower-bud physiological differentiation stage: specifically, total sugar content increased from 15. 43 (-7 d) to 31. 38 mg / g ( 7 d); starch content increased from 5. 42 to 8. 31 mg / g during this time, which may be relevant to the external environment or the original metabolism damaged by potassium chlorate.