The thermocline heat storage tank can improve the peak shaving capacity of the combined heat and power (CHP) units during the heating period, so it is gradually toward large-scale direction. However, the design parameters have a great impact on the performance of the heat storage tank, and the current performance evaluation methods are inefficient. For this reason, the physical model and mathematical model of a large-scale heat storage tank were established, the formation and change process of the thermocline in the process of heat storage were studied. Then, the concept of the equivalent volume of the thermocline was proposed, and the influence of structural parameters and operating parameters on the equivalent volume of the thermocline was analyzed. The results show that after the formation of a stable thermocline, the thickness of the inclined temperature layer changes little with the increase of the heat storage capacity. Thus, the equivalent volume of the thermocline can be used to evaluate the performance of the heat storage tank more efficiently. Comparing the relative variation of the equivalent volume of the heat storage tank under different working conditions, it can be seen that the influence of different influencing factors on the equivalent volume of the thermocline is ranked from high to low as follows: water distributor arrangement, heat storage flow, height-diameter ratio and temperature difference between cold and hot water. The research results provide a new reference for the performance evaluation of large-scale heat storage tanks.