Oyster shell was used as alkalinity supplementary material to study the feasibility of supplementing alkalinity with oyster shell in biofloc technology-tilapia(GIFT Oreochromis niloticus)culture system. At the start-up stage, the feasibility of supplementing alkalinity with different oyster shell dosages of 0 g/L (control group, group A), 0.36 g/L (group B) and 0.72 g/L (group C) was evaluated. The results showed that the level of alkalinity, pH and calcium in group C were significantly higher than those in group A, but there was no significant difference between groups B and C, so the effect of supplement alkalinity with oyster shell is obvious. Based on the results of the start-up of biofloc system, the effects of supplementing alkalinity to tilapia culture system with two forms of oyster shell (shell powder, group E) and (shell, group F) were studied. There were no significant differences in water quality, digestive enzyme activity, immunity and bacterial community composition among the three groups. Oyster shell and shell powder had no significant negative effects on the growth of tilapia and could be applied to the biofloc aquaculture system, but there is no obvious effect of supplementing alkalinity with oyster shell in culture stage. The supplementation of alkalinity had nothing to do with forms of oyster shell. Under the current experimental conditions, oyster shell can not completely replace sodium bicarbonate in the biofloc aquaculture system, and the related technology needs to be further optimized.