With the rice sugary endosperm mutant (Sug-11) and its wild-type (Zhonghua 11) as materials, the total soluble sugar, sucrose contents, starch accumulation in filling grains and the starch physicochemical properties were compared to further reveal the temporal patterns of sugar content, several key enzymes activities and their isoform genes transcriptional expression level during the grain filling so as to analyze the physiological mechanism for Sug-11 whose sugars contents change and 1000-grain weight decline from the point of starch biosynthesis pathway in rice grains. The result showed that the differences in soluble sugar and sucrose contents between Sug-11 and its wild-type were not obvious at the initial stage of grain filling, but with the filling process it gradually became apparent. Secondly, the amylose(Am) content and BV(Am) of Sug-11 were significantly decreased while the starch solubility and BV(amylopectin, Ap) were increased compared with Zhonghua 11, indicating that the physicochemical properties of grain starch were significantly affected in the sugary endosperm mutant. Finally, among the key enzymes in starch synthetic metabolism, debranching enzyme(DBE), existed in Sug-11 seeds, were quite different from its wild type in activity and dynamic in the process of grain filling, and PUL transcriptional expression level and DBE activity strikingly decreased at the middle-late stage, resulting in the declined starch accumulation and increased sugar content in Sug-11. However the poor filling and 1000-grain weight decline were, to some extent, associated with dropped ADPGase activity at the early grain filling stage.