In order to study the character of heat storage and temperature regulation, a mathematical model of the thermal performance of an ideal passive energy storage building is established. Indoor thermal environment evaluation indexes of the passive buildings such as the temperature attenuation coefficient and supercooling degree/superheating degree are proposed. The PCM (phase change materials) layer numbers, the total thickness and the thickness ratio of PCM layers of the combined PCM envelope are analyzed based on the simulation. The result shows that the combined envelope with two-layer PCM has better temperature regulation and heat storage effect. When the total thickness of the combined PCM envelope is 40 mm, and the thickness ratio of the PCM layer from the inside to the outside is13.3:26.7, the annual thermal comfort hours are the longest.