Abstract: We examined the influence of abrupt changes of salinity from 18 to 3, 25, 40 at 24^th hour and 48^th hour on superoxide dismutase (SOD), alkaline phosphatase (AKP), catalase (CAT) and lysozyme (LSZ) of Crassostrea hongkongensis hemolymph, so as to investigate the effects of salinity stress on these immune factors. Results indicate that the changes of four immune factors are not significant (P>0.05) in control group (salinity 18). Changes of AKP and LSZ vitality of salinity 25 group are much higher than those in the control group (P < 0.01) at 24^th hour. SOD activity of salinity 3 group is much higher than that of the control group (P < 0.01) at 24^th hour, while the other factors show no significant difference compared with the control. In salinity 40 group, the SOD activity is significantly lower than that of the control group (P < 0.01) at 24^th hour, and LSZ activity has no significant difference at 24^th hour compared with the control, while sharply declines and becomes significantly lower than the control group (P < 0.01) at 48^th hour; there are no significant difference of other factors compared with the control. The results suggest that salinity may stimulate the immune activity significantly and enhance the body′s immune activity at a certain range. In addition, oyster has strong ability to adapt to low salinity, but in higher salinity stress, its immune activities become lower.