Based on the seasonal field investigations from July 2017 to April 2018, seasonal variations in the in-situ proliferation (growth) of Microcystis using frequency of dividing cells (FDC) technique and their influencing factors were studied in the nearshore water of southern Lake Erhai, Southwest China. Our results showed that the FDC of Microcystis ranged from 6.13% to 29.23%, which was significantly greater during the day than at night, while the differences both between the two sampling sites and among the three sampling depths (i.e., surface, 0.5 m; middle, 1.0 m; bottom, 1.5 m) were not significant. The mean in-situ growth rate of Microcystis was calculated to be (0.36±0.06) d^-1, which was generally higher than that of other lakes that had been studied. Meanwhile, both the FDC and the in-situ growth rate showed significant seasonal variations, i.e., the largest in winter (January, 19.65%±4.10%, (0.39±0.01) d^-1), followed by autumn (October, 14.48%±4.73%, (0.36±0.02) d^-1) and summer (July, 12.77%±3.81%, (0.37±0.07) d^-1), and the smallest in spring (April, 10.37%±2.64%, (0.30±0.06) d^-1), which was mostly due to the important influences of surface solar radiation downwards, and water temperature, etc., whereas the nutrient concentrations did not play significant roles. Furthermore, except the key factor surface solar radiation downwards, other major environmental factors affecting the diurnal changes of FDC had seasonal heterogeneities to some extent. Our results can provide a useful information for further studying the occurrence mechanism of cyanobacterial (Microcystis) bloom as well as its prediction and early warning in Lake Erhai.