Reproductive molt has special significance for Brachyura because it determines crab’s two different growth phases for postlarval development: the puberal phase and the mature phase and only after this molt, the crab has the ability to mate. However, the swimming crab Portunus trituberculatus, one of most important marine economic species in China, the process of reproductive molt and mating still has not been observed in detail. Our study recorded this process and relative mating behaviors under the laboratory-reard condition based on the daily external characteristic examination and real-time infrared videography in the mating season. As the prepubertal female progressed towards the reproductive molt, the abdomen of female P. trituberculatus changed from faint shades of blue to dark blue that eventually covered the entire abdomen while the propodus of the swimming leg finally formed a red crescent. These female particular external characteristics appear in pre-molt stage and therefore, may be as one kind of visual signals for subsequent mating activitie as pheromone. The mating process started from the behavior of courtship embrace while the copulation occurred immediately after reproductive molt. It was a continuous course including following post-copulatory guarding embrace. But compared with the prolonged pre-and/or post-copulatory embrace, the time used for molting and mating was very limited. So this may be a reproductive strategy adopted by P. trituberculatus: for one thing, the female can make her reproductive molt and soft-shell mating completed safely and orderly within a limited time by mean of embrace; for another thing, as compensation, male can quarantee paternity by formation of sperm plug in the female reproductive tract to reduce mating chance and sperm competition.