By means of the observational data of the planetary boundary layer observation system in the Dali national climate observatory, the late frost event occurred on 14 March 2009 in Dali is analyzed. The main results indicate: (1) The late frost injury occurred after rainfall and warmer weather, resulted from upperlevel cold advection and radiation cooling at night. (2) Before the frost, the meteorological elements showed significant changes due to the near surface layer affected by cold air. The main variation features during the day include high atmospheric transparency, cloudless sky, small wind speed, high air pressure, low air humidity, strong solar radiation on the surface, more surface heat storage, and higher temperature and maximum time delay. The latent heat flux played a dominant role in heat transferring. After sunset, the surface heat began to release, and the released heat was more than the absorbed during the day. Watervapor content and longwave radiation were stable, and air temperature decreased rapidly, and there appeared condensation phenomenon near the ground.