| The reproductive biology of Urechis unicinctus was studied by the method of experimental ecology and histology, including the sex ratio, gonadosomatic index (GSI), fecundity, embryonic and postembryonal development. The optimal environmental conditions for fertility and hatching rate of U. unicinctus were also assessed by orthogonal experiments in order to provide reliable protocols for the artificial culture of U. unicinctus. The sexual ratio between the female and male was 1:1.17 and the males were slightly more than females. The mature oocyte remained at the prophase of the first maturation division with the diameter of (145.16±1.70) μm × (141.27±1.94) μm. The absolute fecundity ranging from 111 800 to 720 488 (449 684±265 905), while the relative fecundity ranging from 2 214 to 12 702 (6 849±4 854), and positive correlation between fecundity and weight were recorded. The biological minimum was about 7 cm of length and 11.21 g of weight. The sperm density was (5.88±0.52) × 109/mL, and the average number of sperm produced by each male was (2.41±0.73) × 1010. Two peaks of female GSI were observed in the breeding season from April to May, which suggested the type of spawn of U. unicinctus was in batches. The trochophore was hatched after 20-24 h of embryo development under the condition of salinity 25 and temperature 15 °C. According the histological observation, the oocyte of U. unicinctus was a type of mesopodium that developed into hemipolar blastocyst by unequal cleavage and spiral cleavage, and then the gastrulation was formed via outsourcing and invagination of cell migration. The postembryonal development after hatching included three stages: trochophore (1-10 d), somatic larva (11-20 d) and worm larva (21-30 d). The results of orthogonal experiments showed that the highest fertility and hatching rate was obtained under the conditions of temperature of 25 °C, the salinity of 35, and pH 8～9, and temperature of 15 °C, the salinity of 25, and pH 8～9, respectively.