Different from the high latitudes (24°N–53°N) of China, the low latitudes (4°N–24°N) present large solar elevation angle and strong solar radiation throughout the year. Radiation differences in all directions are smaller than that in high latitudes, which results in the building envelope optimization (BEO) in high latitudes is not applicable to low latitudes. Therefore, for the low latitudes, this research calculated the energy-saving effect of buildings with different shape parameters and the cooling load of thermal performance of different envelope by numerical simulation. In addition, the corresponding energy-saving indexes for cooling load reduction (CLR) are presented. And the energy saving priority of building envelope is mastered by analyzing the percentage of CLR per unit volume, the roofs have the greatest energy saving potential, followed by exterior walls, sunshade and exterior windows. Furthermore, the recommended insulation thickness for the exterior walls and roofs should be 30–40 mm, and of which the CLR value of per unit external wall area can reach 8.5 W/m2. The west exterior window has greater energy saving potential than other orientations. The effect of east and west sunshade on CLR is much greater than that of south sunshade. The recommended length of the north and south shading should be 500–600 mm, in which the cooling load from shading can be reduced by 95％ and 15％ respectively. The results can be the basis of the design method of the BEO in low latitudes of China.