Climatic characteristics and life characteristics of southwest heat low are analyzed through survey from March to May of 1997-2006. In view of synoptic point, it can be divided into four developing stages: primary, mature, maintenance and decay. Based on the station observational data as well as NCEP reanalysis data, the typical southwest heat lows in spring are analyzed and the structural characteristics are revealed. The diagnostic results show that under the control of southwest heat low in spring, pressure decreasing and temperature increasing are significant, and meteorological elements have obviously diurnal variations. Southwest heat low is a shallow warm low, but with deep dry zone. The low zone is characterized by convergence and positive vorticity. This system is of convective instability.