The reproductive system of Chinese mitten crab, Eriocheir sinensis is complicated, and accessory gland, testis and vas deferens are three major constituents. However, no available information could be found on the biochemical composition of these three tissues for the mature male E. sinensis. Therefore, this study was conducted to investigate the proportion, proximate composition, fatty acid composition and amino acid contents in the reproductive system of mature male E. sinensis. The results showed that:(1) the accessory gland index (AGI) was significantly higher compared to the testis index (TI) and vas deferens index (VDI) (P<0.05). (2) For the biochemical composition, the contents of moisture, total lipid and total carbohydrate in the testis were significantly higher than those of the other parts (P<0.05), while the highest crude protein content was detected in the accessory gland (P<0.05). (3) For fatty acids profile, no significant differences were found in the percentages of the most saturated fatty acid (SFA), total saturated fatty acid (ΣSFA) and monounsaturated fatty acid (MUFA) among three parts, while the testis had the highest levels of C20:3n3, C20:4n6, C22:6n3, total polyunsaturated fatty acids (∑PUFA), ∑n-6PUFA and total highly unsaturated fatty acids (∑HUFA) (P<0.05). (4) For amino acids composition, the highest contents of the major amino acids were detected in accessory gland, while the contents of L-lysine (Lys), valine (Val) and Histidine (His) were shown as VD > AG > T (P<0.05). In conclusion, the high contents of total lipid, total carbohydrate, PUFA and HUFA were stored in the testis of mature E. sinensis, while the accessory gland had the highest contents of crude protein and total amino acids. These differences may indicate their different physiological function for these tissues of mature E. sinensis.