To discuss the effect of energy saving of heat-reflective insulation coating on exterior walls of building, the wall temperature change of two test rooms with heat insulation or not in Hangzhou has been monitored in summer, and the heat flux data of east, south and west walls were analyzed. The results indicate that the temperature of outside wall surface with the heat reflective insulation coating is reduced effectively, and the maximum temperature change was about 9, 10°C. There is a positive proportional between the mean daily air temperature and heat gain that the heat gain increases with the mean daily air temperature. By calculating, the higher the air temperature, the more the cooling effect of coating in summer. But the higher the mean daily temperature do not mean that the more heat gain in summer. The result shows the energy saving effect of the heat insulation coating on building wall was very obvious in summer in Hangzhou.