Palaemon annandalei is a small commercial shrimp species in China. Palaemon annandalei is mainly distributed in the Yellow Sea, East China Sea, and western coast of the Korean Peninsula. Since P. annandalei is the dominant macroinvertebrate species in northern Hangzhou Bay, this species plays an important role in marine ecosystem balance in this area. Hangzhou Bay is a traditional feeding and spawning ground for many fishes, such as Collichthys lucidus and Coilia mystus. Palaemon annandalei is also one of the prey items of C. lucidus in nor-thern Hangzhou Bay, and is important for C. lucidus growth and breeding. Based on data from 12 set-net surveys in northern Hangzhou Bay in February, May, August, and November 2018, power function, relative body weight, one-way ANOVA, and covariance analysis were applied in this study to analyze the body length-weight relationships (LWR) and relative fatness of P. annandalei. The results showed that the impact of seasonal changes on P. annandalei body length and weight compositions was statistically significant. Mean body length and body weight decreased from spring to winter, and seasonal changes on P. annandalei mean body length and weight was statistically significant (P<0.05). The condition factor (a) of LWR was highest in summer and lowest in winter, and firstly increased, then decreased from spring to winter. The allometric factor (b) of LWR was highest in winter and lowest in summer, which presented opposite trends to a. Covariance analysis showed that a and b was not significantly different between spring and summer (P>0.05), and all the other seasons had significant differences between each other (P<0.05). The relative fatness of P. annandalei increased from spring to autumn, reaching a maximum in autumn, then decreased to the minimum in winter. A one-way ANOVA showed that relative fatness was not significantly different between spring and summer (P>0.05), and all the other seasons had significant differences between each other (P<0.05). The research indicated that seasonal changes on a and b of LWR of P. annandalei in northern Hangzhou Bay were statistically significant, and the changes on a and b of LWR was mainly affected by predation, fishing pressure, and environmental factors.