The immune activity of mollusk is correlated with seasonal changes and reproductive cycle. Mass mortality of mollusk is often observed in certain seasons when the mollusk’s immune activity depresses. Herein, the study investigated the seasonal effects on immune parameters of the broodstock of small abalone, Haliotis diversicolor, and the correlation between the parameters and environmental variables from March, 2008 to May, 2009. The results indicated that the total hemolymph protein, total hemocyte count, hemocyanin, and activities of alkaline phosphatase and pheoloxidase in hemolymph during the reproductive season, October to January, were lower than those during the non-reproductive season. Result of the Pearson correlation analysis showed that most of the immune parameters were positive correlated with water temperature (P < 0.05). Based on the results, it suggested that water temperature and gonadal maturation could produce synergistic impacts on immune parameters.