Abstract: The lake effect of Erhai in the complex valley and basin area was simulated using the WRF_CLM model which includes a lake module. The observational stations over cropland and lake were used to verify the model results. We have analyzed the lake effect of Erhai on local circulation and boundary layer structure during monsoon and non-monsoon periods with the model. The experiments with or without Erhai lake during non-monsoon period show that Erhai lake has a great impact on the local circulation and boundary layer structure. Compared with land surface, lake surface transfers less turbulent flux, and the development of turbulence is weaker in the daytime, which leads to lower boundary height. Valley breeze only reaches up to 200 m without the lake breeze. There is no mountain-valley breeze circulation in the nolake experiment. In the evening, the strong synergism of mountain breeze of Cangshan and Yu'anshan makes the circulation height reaches up to 600 m.Affected by precipitation system, local circulation can't develop well during the monsoon period. Divergence flow during the daytime and cyclonic circulation during the nighttime are weaker in monsoon periods than that in non-monsoon periods. In the daytime, there are clouds formed, and the boundary layer height is low. In the evening, the difference of water vapor pressure between lake surface and air is large, which results to the great value of latent heat flux. At the same time, turbulance developes, and boundary layer height even gets higher than that in the daytime.