Palaemon annandalei is a commercially important shrimp in China, which is mainly distributed in the Yellow Sea, the East China Sea and the western coast of the Korean Peninsula. P. annandalei are especially abundant at the Yangtze Estuary and its adjacent waters, which are the main catches by set net in northern Hangzhou Bay. A monthly sampling of P. annandalei by set net in northern Hangzhou Bay during the period from April 2017 to March 2018 was conducted for studying on its reproductive biology. The methods using to analyze these captures included curve estimation, chi square test, independent sample T test and paired sample T test. We analyzed the reproductive biological index such as body length, weight and sex ratio et al. of P. annandalei and got the results as the following: the distribution of body length of the shrimp ranged from 14 to 51 mm, whereas the average body length was 32.0 ± 5.4 mm, n = 3 210. The body length ranged from 27 to 51 mm, whereas the average body length was 36.3 ± 3.4 mm, n = 375. The body weight ranged from 0.016 to 1.700 g, whereas the average body weight was 0.565 ± 0.282 g, n = 3 210. The body weight of spawning stock ranged from 0.400 to 1.700 g with a mean body weight of 0.868 ± 0.225 g, n = 375. There was a significant difference (P < 0.01) between female and male on average body length and average body weight. The sex ratio of female to male was 1.27︰1, females were significantly more than that of males (P < 0.01). The reproductive period of the shrim in northern Hangzhou Bay lasted from late April to late October with two peak periods in June and August; a significant linear correlation was detected between brood porportion (B) and sex ratio (S) (P < 0.05), for the relational expression was: B = 41.80S﹣21.06 (R2 = 73.9). The absolute fecundity (FA, egg) ranged from 17 to 179 eggs, and the average absolute fecundity was 60 ± 26 eggs, while the average relative fecundity (FR , egg) was 70 ± 24 eggs/g, n = 375. The absolute fecundity was significantly correlated with body length (L, mm) and body weight (W, g) by quadratic function (P < 0.01), and the two relational expressions were FA = 0.29L2﹣18.52L + 342.67 (R2 = 76.8) and FA = 97.32W2﹣100.75W + 71.66 (R2 = 94.5), respectively. The long egg diameter ranged from 1.090 to 2.688 mm with a mean of 1.599 ± 0.158 mm; whereas the short egg diameter ranged from 0.807 to 1.992 mm with a mean value of 1.141 ± 0.118 mm, n = 2 787. Comparing our data with those collected in the 1980s, we found out that the body size of P. annandalei has miniaturized, while the shrimp has prolonged its reproduction period from single peak to double peak and increased its individual fecundity ability, as results from both environment degredation and higher fishing pressure.