Microcystis aeraginosa is cultivated in BG-11 medium and nature water for 100 d. Dynamic law of microcystic toxins MC-LR released by microcystis aeraginosa in different growth phrases is studied. The results show: the chlorophyll-a concentration maximum of microcystis aeraginosa appears 10 d ealier than the MC-LR concentration maximum, whatever algal is cultivated in BG-11 culture medium and nature water. Similar dynamic laws showe in both culture medium are that chlorophyll-a concentration and MC-LR concentration are positively corelated in the logarithmic period and negatively correlated in the stabilization period, and chlorophyll-a concentration reduces while MC-LR concentration first increases, then decreases in the decline period. This regularity maybe closely linked to the characteristics that pigment in vivo cells released into medium could catalytic MC-LR photodegradation. The linear relationship from fitting between the chlorophyll-a concentration and MC-LR concentration, of purebred microcystis aeraginosa cultivated in laboratory condition, is used to provide a basis of approximately estimating microcystic toxins MC-LR concentration by chlorophyll-a concentration of water.