The experiment included the field investigation and laboratory test to explore the omechanism of Microcystis toxin production. We collected Microcystis waterblooms in the three ponds and analyosed water samples and toxicity bioassay from April to Novemeber in 1987. Microcystis waterblooms in the two of the three ponds are toxic and the other was non-toxic from the beginning to the end. Toxicity of Microcystis was recongnized as the existence of toxic strain. Toxic strain of Microcytis waterblooms were made of two kinds. Comparison between changes in environment facters and toxicity showed that toxicity varied with different seasons. Toxicity decreased with the increase of the temperature and it also changed with different concentrations of N, P elements in the water, but N, P were net the key factors to centrel the toxin production of Microcystis waterblooms. Laboratory test was to observe the effect of the N, P concentrations on toxicity of the toxic strain M.8641 and non-toxic strain M.8508 in culture with the basic medium of Chu10. The results showed that N is the limit factor to affect Microcystis growth compared to P. Toxicity of M.8641 increased during exponential phase. Laboratory test also stated N, P were not the key factors to control toxin production of Microcystis.