In order to clarifying the impact of temperature reduction on the decline and sedimentation of Microcystis, the decline process of Microcystis bloom in Gonghu Lake was monitored and low temperature culture experiments were conducted. The results show that the laboratory simulation results are basically consistent with the phenomenon in the field in autumn and winter. Microcystis generally declines at temperature below 15℃ and sinks in large numbers below 12.5℃. The temperature reduction leads to a significant decrease in the size of the Microcystis colonies, which is related to the number of dead algal cells within the colony. When the mortality rate reaches 25%~45%, the large colonies will generally begin to disintegrate and disperse into small colonies. Gravity density of Microcystis continues to increase at low temperatures, and it eventually loses its ability to autonomously float to the surface. The initial decline in sedimentation of Microcystis in autumn and winter is mainly caused by colonies miniaturization and loosening, and the increase of gravity density is likely to be the main cause of Microcystis sedimentation in the later stages of temperature reduction. Keywords: low temperature; Microcystis ; decline ; colony; gravity density; Gonghu Lake