Abstract: In the process of solar energy utilization, the thermal storage water tank is the crucial element to improve the thermal efficiency and the capacity of heat storage. In this paper, sodium acetate trihydrate was selected as the latent storage material, the storage tank test platform was designed and constructed. The CFD model of the hot water storage tank was established, and the heat release process of the hot storage tank was simulated, combined with the experimental results to deeply analyze the thermal stratification mechanism of the water tank. Under the conditions of initial water temperature of 353.15 K and inlet water temperature of 278.15 K, their influence on the temperature of the heat storage tank was measured with dimensionless time in different positions of phase change material (PCM) sphere and flow rates. The Richardson number and mixing (MIX) number were used to extensively analyses the thermal characteristics of the heat storage water tank with different position as the variation in flow rates. The results show that the lower the position of the PCM, the smaller the inlet flow rate of the water tank, the MIX number, and thickness of the thermocline layer are smaller, and the Richardson number is larger. The half-life of the PCM sphere (i.e., the liquefaction rate reaches 50%) is prolonged as the inlet flow rate increases. The simulated values of the Richardson number are slightly higher than the experimental values, while the simulated values of the Mix number are less than the experimental values. The RMSE of Richardson, MIX number, and simulation value increases with the decrease of the position of phase change heat storage sphere and the increase of inlet water flow.