In order to investigate the effect of Bellamy aquarata size on the predatory behavior of Eriocheir sinensis, the differences of size selection, predation rate, predation behavior and shell-breaking techniques of small and large sizes[shell width:(38.0±2) mm; (43.0±2) mm] Eriocheir sinensis preying on three sizes[shell width:(5.5±0.5) mm, (7.5±0.5) mm, (9.5±0.5) mm] were analyzed by video surveillance system. The results were as follows:The larger the size of B. aquarata, the lower the selection index and predation rate of E. sinensis. (P<0.05). The probability of capture upon encounter and the probability of consumption upon capture of E. sinensis and B. aquarata are consistent with the trend of predation rate, but encounter rate of E. sinensis and B. aquarata is opposite to the trend of predation rate; The smaller the size of B. aquarata, the greater the probability of consumption upon capture of B. aquarata (P<0.05). B. aquarata size significantly affected the proportion of stationary time, moving time and handling time (P<0.05), but had no significant effect on the proportion of searching time (P>0.05). The larger the size of B. aquarata, the higher the proportion of successful avoidance (P<0.05); the larger the size of E. sinensis, the lower the proportion of successful avoidance of B. aquarata (P<0.05). The proportion of crushing tactics of E. sinensis on small and medium sized B. aquarata was significantly higher than that on large sized B. aquarata (P<0.05). E. sinensis preferred to prey on small-sized B. aquarata. Whether the crab can consume after capture is an important link in the predation of E. sinensis. Shell-breaking techniques of the E. sinensis to the square ring edge snail change with the size of the square ring edge snail.