Abstract: We examined the influence of abrupt changes of salinity from 18 to 3, 25, 40at 24th hourand 48th houronsuperoxide 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 (P0.05) in control group (salinity 18). Changes of AKP and LSZ vitality of salinity 25 group are much higher than those in the control group (P0.01) at 24th hour. SOD activity of salinity 3 group is much higher than that of the control group (P0.01) at 24th 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 (P0.01) at 24th hour, and LSZ activity has no significant difference at 24th hourcompared with the control, while sharply declines and becomes significantly lower than the control group (P0.01) at 48th 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.