The air layer in expanded polystyrene(EPS) external insulation walls has an important effect on its coupled heat and moisture transfer. Considered the climate in the hot summer and cold winter area such as Shanghai, the numerical simulation of the coupled heat and moisture transfer was carried out for EPS external insulation perforated brick wall including an air layer of different thickness by using the CHAMPS BES software. The results show that the moisture in the perforated brick transfers towards the air layer more quickly with thickening the air layer. Also the peak moisture content of the EPS board and the air layer is higher and it takes more time for moisture to reach stable with thickening the air layer. The moisture has a more remarkable effect on the heat transfer coefficient of the EPS external insulation perforated brick wall with thickening the air layer. When the thickness of the air layer is 20 mm, the heat transfer coefficient of the EPS external insulation perforated brick wall is about 34% greater than the steady heat transfer coefficient. It is best to decrease the thickness of the air layer during the construction, provided that the bond strength between the EPS board and wall body can be fully guaranteed.