Changes of the gonad, survival rate, and life span of the precocious (i.e., sexually mature in their first October–November, at the age of 5 or 6 months) Chinese mitten crab (Eriocheir sinensis) were studied in a small, shallow, macrophytic, freshwater lake along the middle reaches of the Changjiang River, China. The gonosomatic index (GSI) reached peak during the next March when the female GSI was 12.32 ± 1.75 SD% and the male GSI was 4.24 ± 0.19%. The sexual glands degraded from then on. In the first ten-day period of the following July, there was no complete ovum in the ovary, and the sperm became thin and lost adhesion. The population declined sharply from November to July, and the last one (a female) died in the middle of July, which indicated that the life span of the precocious crab was about 12 months (from larval hatching in June to death in July of next year). The survival curve might be expressed as Y = 1.09exp(−0.018x) (Y: survival rate; x: days) for the precocious crabs stocked in experimental cages.