Abstract: The activity levels of superoxide dismutase (SOD), catalase (CAT), lysozyme (LZM), acid phosphatase (ACP), and alkaline phosphatase (AKP) in the clam Potamocorbula laevis were measured, while the seawater temperature was rapidly decreased to freezing point from 7.50℃, 4.25℃, 1.95℃ and kept 24 h. The enzyme activity has been decomposed into the baseline value at different temperature levels, and the enzyme activity for stress response on temperature plummeting and sustained low temperature, which based on the theory of ecological immunology. Then, the decomposed enzyme activities were expressed using suitable mathematical expressions. The stress response course of enzyme activity with time was fitted using the expression superimposed by 3 components expression. It is found that there was same change trends among the five enzymes. The fluctuation that was leaded by temperature plummeting was occurred earlier than that was caused by sustained low temperature, and the peak value of fluctuation in the former was higher than that in the latter. The method can present full process to compensate for the imperfection of histogram and provide activity levels and appearance time of peak value.