The thermal inertia index D is the key parameter in the thermal stability model of the building based on the harmonic analysis method. To reveal the effect mechanism of D on the heat flow fluctuation in the enclosure structure, the relationship between D and the attenuation and the number of temperature waves was given, based on the theoretical analysis and solution of the heat transfer process of the building envelope under the periodic unsteady thermal action. Then the effect of D on the attenuation and delay of temperature wave in the enclosure structure is analyzed by numerical simulation method. Finally, the key influence of D on the calculation of thermal stability parameters was analyzed, such as violent fluctuating layer thickness, the thermal storage coefficient of inner surface, the attenuation multiplier and the delay time. The results show that there is a quantitative relationship between D and the attenuation of temperature wave and the number of temperature waves in the enclosure. For the same type of enclosure structure, when D value increases, the attenuation and the delay time become greater. As the number of temperature waves in the enclosure structure increases, and the thermal stability become better. For different forms of multilayer enclosure structure, the larger the D value is, the longer the delay time is, and the attenuation ratio is related to the arrangement of the material layer, and the external insulation can obtain more attenuation multiple. Using D=1 to determine the thickness of violent fluctuating layer provides a new idea for thermal storage design of building envelope. The calculation of the thermal storage coefficient of inner surface is mainly related to the thickness of violent fluctuating layer. The theoretical basis for the improvement of the physical meaning of the thermal inertia index from the qualitative description to the quantitative description is provided.