Abstract: The compared analysis for two tropical depressions in South China Sea is completed by using numerical model developed in Guangdong Institute of Tropical Marine Meteorology and ECMWF data. One of them developed into a typhoon in 24 hours, but another undeveloped. The analyses show that their initial temperature, humidity, wind and pressure fields all have different structure obviously. Especially, the developed depression has a deep high-humidity layer and a clear warm-cored structure in the lower troposphere, the conditional unstable layer is deeper; the undeveloped depression has not clear warm-cored in the lower troposphere and the humidity in the mid-lower tropo-sphere is smaller, the conditional unstable layer is thinner. Different developing tendencies of two depressions are simulated successfully. Based on the model output and their comparison, the fundamental physical process which leads the tropical depression to the development or the undevelopment is discussed. It is clear that deep high-humidity layer in the mid-lower troposphere ands relevant temperature/wind field structure maintained and enhanced a strong convection heating, and it is very important to the development of the depression.