To enhance our understanding of the mating process of the Chinese mitten crab, Eriocheir sinensis, the morphological functions of the copulatory system were observed and studied using flash-frozen samples, scanning electron microscopy, and histology. The copulatory system had the typical characteristics of Brachyura and belonged to the G1 long type. The male G1 was generally thick and smooth, in the form of a triangular column, with a suture groove formed by the overlap of the lateral margins of the endopodite, which contained a seminal canal and ran through the two ends. The base had one relatively large opening oriented towards the inner side of the dorsal surface, and the outer side was covered with a meat-like valve with feathery setae. During mating, the penis can extend into the outer part of the opening (or through a valve), and the inner part is inserted by G2. The distal end contained the other seminal opening, similar to a volcano mouth with bristle-like setae all around. G2 was small and sail-like, with the tip invaginated and four long transparent setae attached. The walls of both G1/G2 endopodites were different in thickness, and the seminal lumen of G1 corresponding to the G2 insertion part was thin. Therefore, the wall had a certain swelling capacity, and the lumen could be completely closed with the G1/G2 interaction during mating. Most of the G1 endopodite was hollow, and there were loose connective tissues with droplets above the insertable part of G2; correspondingly, many passages connected the lumen on the inner side of the tube wall. It could be inferred that there could be some form of communication between semen and G1 itself. Muscle tissue appeared in the latter half. The vulva was covered with a smooth and thick operculum, and the joint between the operculum and the thoracic segment of the mature individual could be easily shifted left and right to control the opening and closing of the vulva. The vagina was crescent-shaped in cross-section. One side of the chitinous wall was invaginated into the other, and thick muscles were attached to the invaginated wall. Finally, it can be concluded that G1 does not directly insert into the female reproductive tract but directly matches with the operculum and the surrounding thoracic segment. Through the co-evolution of G1 and G2, a “docking” mating system has developed to ensure that the sperm and other male materials are successfully transported into the spermathecae through the vagina for storage. This study described the potential internal cooperation mechanism of the copulatory system of E. sinensis, enriching the biological knowledge regarding crab insemination and providing theoretical and technical support for its genetic breeding and artificial insemination in the future.