Abstract: A mathematical model of heat transfer in solar collector system was established by analyzing the process of heat balance in each subsystem of solar heating system. According to the vaporization’s principle of the internal fluid of collector,the overheating evaluation index was proposed. Overheating degree of the solar collector system under different conditions such as heat flowrate,volume of water tank and collecting tilt angle was analyzed. The results indicate that the overheating degree decreases with the increase of heat flowrate,collecting tilt angle and volume of water tank. Through sensitivity analysis of the above three factors,the heat collecting tilt angle has the greatest influence on system overheating.