Healthy Portunus trituberculatus (averaging body weight 176±21.3g) were used to determine the changes of the immunity enzyme activities in haemolymph, muscle and hepatopancreas tissue after the crabs were injected with 0.9% Nacl solution (Control) and Vibrio alginolyticus. The relative immunity indicators were measured at 0h, 24h, 48h and 72h after the infection. The results showed that: The lysozyme (LSZ) activity of heamolymph increased significantly (P<0.05) in V. alginolyticus infected crabs in the initial 24h, but at 48h and 72h the activity reduced significantly (P<0.05) below the control level. The LSZ activities of muscle and hepatopancreas decreased significantly after infection (P<0.05). Both the ACP and ALP activities reduced significantly in the heamolymph, muscle and hepatopancreas tissue of infection group respectively (P<0.05). However, the control crabs did not experience any significant variation in LSZ, ACP and ALP activities in all the tissues. The results of this study indicated that immunity enzyme in P. trituberculatus infected by V. alginolyticu was induced more significantly, which accompanied by weakened immunity ability of the crabs. 