Based on the existing quality problems of external wall insulation system, a new external wall insulation system was developed through the rational use of closed air layer’s thermal insulation performance. Considering three kinds of ways of heat transfer-- convection, conduction and radiation and using the coefficient of air equivalent thermal conductivity to obtain the thermal resistance in the air layer, compared with the thermal resistance at different thicknesses in the new system using polyurethane and polystyrene board of insulation material with two different thicknesses, the authors can get the most optimal model. This paper takes the extreme value of indoor and outdoor temperature in summer as the temperature loads of this model and uses ANSYS simulation to obtain its heat flow density, gradient and temperature field, and in the meanwhile, undertakes a comparison with the present form of external wall insulation in the new code. Data indicates that in the same index of energy saving, this system can not only meet the needs of the code, but also reduce the requirement of heat insulator, as well as greatly improve anti-cracking and waterproofing ability. Thus, the new system follows the direction of the development in this field.