Abstract: Objectives To find out the rule and distribution of algae toxins in Dianshan lake.Methods Microcystin (MC)-LR in water samples from 5 points in Dianshan lake were examined monthly by LC-MS/MS during March to September in 2013, and the counts of algae cell were also monitored.Results The counts of algae cell had begun to raise since March, and got peak in June (49.20×10⁵/L), then fallen down in July; the counts of algae cell in August was 1.65×10⁵/L in the center of the lake, the average count was 2.51×10⁵/L; The toxic MC-LR could not be detected in March in the center of the lake, which was 72 ng/L in the sports pool in July, and then the toxin began to fall down.Conclusions Blue algae was the dominant algae in Dianshan lake, free MC-LR could be examined, and the peak of MC-LR was emerged a month later of the peak of algae cells counts. The relatively low levels of MC-LR were found in the center of the lake.