Abstract: 【Objective】 To clarify the characteristics of leaf photosynthetic assimilate partitioning to fruit in Wen185 walnut (J. regia Wen185). 【Methods】 In this study, we used the ¹³C isotope pulse labeling technique to label the whole plant of 5-year-old walnut trees and analyzed the changes in the rate of ¹³C partitioning (R¹³C), leaf source strength, and fruit pool strength in fruits and leaves during different fruit development periods. 【Results】 R¹³C gradually decreased in leaves and maintained at a high level from 40 to 60 d after anthesis; R¹³C showed a 'rise-fall-rise-fall' change in fruits, and significantly increased from 40 to 60 d and from 80 to 100 d after anthesis (P<0.05); chlorophyll content and Rubisco carboxylase activity in leaves showed a 'rise to fall' change, and net photosynthetic rate showed a 'rise-fall-rise-fall' change, both of which were significantly higher at 40 to 60 d after flowering (P<0.05); fruit weight and volume both increased significantly at 40 to 60 d and 80 to 100 d after flowering increased (P<0.05). 【Conclusion】 From 40 to 60 d after flowering, most of the photosynthetic assimilates are used for the rapid growth of leaves, and the rapid growth of leaves is accompanied by the elevation of photosynthetic capacity, at which time the leaves complete the transformation from reservoir to source; from 40 to 60 d after flowering, and from 80 to 100 d, which are the two periods with more allocation of photosynthetic assimilates to the fruits, in which the fruits are growing rapidly from 40 to 60 d after flowering, and the reservoir strength is elevated, and photosynthetic assimilates are mainly used for fruit cell division and morphology construction. In the period from 40 to 60 d after flowering, the fruit grew rapidly and the reservoir strength increased, and the photosynthetically assimilated substances are mainly used for fruit cell division and morphology construction.