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Thermal storage performance is the key to improve the overall efficiency of solar hot water utilization systems. In order to find the optimal structure of solar water storage tank to adapt to the actual situation of solar energy resources in Urumqi, and to determine the optimal operating conditions in different periods of time, the flow and heat transfer processes in solar water storage tank under 25 different working conditions were numerically analyzed by standard k-ε model in this paper. The results showed that the thermal stratification effect of the solar water storage tank with the flat roof was the worst one, the dimensionless exergy was 0.903, the solar water storage tank with the spherical foof was the best and the dimensionless exergy was 0.881. The solar water storage tank with flat roof had the highest instantaneous heat transfer efficiency and its value was 0.899, while the solar water storage tank with the spherical roof had the lowest value 0.855. At 11:30, 13:00, 15:00, 16:20 and 18:00, the solar water storage tank with spherical roof could obtain the optimal thermal stratification effect when the cold water inlet velocity were 0.42, 0.42, 0.1, 0.18 and 0.26 m/s, respectively. The minimum dimensionless exergy for the five times were 0.879, 0.873, 0.879, 0.881and 0.874, respectively. For the five studied times, however, the solar water storage tank with spherical roof had the maximum instantaneous heat transfer efficiency when the cold water inlet flow velocity was 0.10 m/s. And the maximum instantaneous heat transfer efficiency for the five times were 0.864, 0.852, 0.864, 0.865 and 0.862, respectively. If the solar water storage tank was used for heating, the lower limit of cold water inlet velocity of the tank was 0.26 m/s. When the inlet velocity of cold water was under 0.26 m/s, the parallel operation mode of water tank or combined operation mode of water tank and electric heating boiler should be adopted. This study can provide reference for the operation optimization of solar energy storage system in different climate conditions. [ABSTRACT FROM AUTHOR]