Abstract: Heat storage is the key technology of passive solar utilization and heat protection design in summer,and accurate evaluation method or index are the foundation of heat storage design and utilization. The thermal insulation layer has a significant impact on the thermal storage performance of the wall. This article uses experiments and software simulation methods to investigate the heat storage characteristics of the external thermal insulation wall and reasonable heat storage performance evaluation index. Through field tests,three external wall surfaces and internal temperature decrement laws with different orientations have been obtained that due to the thermal insulation effect of the thermal insulation layer,the temperature fluctuations on the internal and external surfaces of the external wall structural layer are small,and the heat storage effect of the structure layers are significantly weakened. With the help of Kvalue software,the relationship between the thermal inertia index and the total degree of decrement of the single-material wall and the external thermal insulation composite wall are simulated and calculated. The results show that for a single-material external wall,there is a good correlation between the thermal inertia index and the total degree of decrement,while for the external thermal insulation composite wall,the correlation between the two is poor. For the external thermal insulation composite wall,a modification method for calculating the thermal inertia index of the multi-layer envelope structure is proposed. The correlation coefficient R2between the thermal inertia index obtained by this method and the total degree of decrement of the wall is over 0.9.