Abstract: We investigated the effects of different concentrations of guava leaf water-extract (GLWE) on the survival rate of Scylla paramamosain carrying virus and its influence on alkaline phosphatase (AKP), acid phosphatase (ACP), lysozyme (LZM), superoxide dismutase (SOD), catalase (CAT), phenol oxidase (PO), total nitric oxide synthase (TNOS) and inducible nitric oxide synthase (iNOS) at different GLWE mass concentrations of 0 (Group C0, control group), 10 mg·L^–1(Group C10), 20 mg·L^–1(Group C20), 40 mg·L^–1(Group C40), 80 mg·L^–1(Group C80) on 0^th, 12^th, 24^th, 48^th, 72^nd hour, so as to study the suitable drug concentration for GLWE. The results show that the survival rate of S.paramamosain increased at first and then decreased with increasing concentration of GLWE, which was significantly different with Group C0 after 15 d (P<0.05). The survival rate of Group C40 was the highest (55.56%), but the survival rate of Group C0 was the lowest (22.22%). The activities of AKP, CAT, PO and iNOS in Group C80 were significantly higher than those in Group C0 on 12^th hour (P<0.05). The activities of SOD and LZM in Group C20 and Group C40, as well as the ACP, CAT and PO in Group C40 were all significantly higher than those in Group C0 on 24^th hour (P<0.05). The activities of CAT in Group C10 and Group C20, as well as the TNOS and iNOS in Group C40 and Group C80 were all significantly higher than those in Group C0 group on 48^th hour (P<0.05). The activities of AKP in all treatment groups were significantly higher than those in Group C0, but the ACP and CAT in Group C40 and Group C80 were significantly lower than those in Group C0 on 72^nd hour (P<0.05). Under this condition, 10–40 mg·L^–1 GLWE could significantly increase the activities of AKP, ACP, SOD, CAT, PO, NOS and LZM in S.paramamosain serum within 48 h, and GLWE of 20–40 mg·L^–1 obtained the optimal effect.