Abstract: This paper deals with the effects of low PH on the survival rate of the embryo, fry and fingerling and the pathologic changes of the fish gill. Under the condition of the hard water, embryonic development of Misgurnus anguillicaudatus was postponed at low PH levels and growth of the fry was also restrained at PH≤5.5. Under the condition of the soft water, the survival rate of fry and fingerling of Ctenopharyngodon idellus would be effected at PH≤4.5. The toxicologic action on the fish had Been strengthened due to the synergistic effects of low PH with aluminium. Pathologic changes including the increase of mucus, swell and disengagement of the lamellar epithelim, hyperplasia and fusion of the tissue were observed. A discussion on the toxicologic mechanism of low PH is proposed.