In solar water heating systems, the structures of thermal storage devices have played essential roles in the improvement of thermal charging efficiency and system performance. This article was focused on the optimization of thermal storage tanks, as well as the influences of thermal storage tank structures on the temperature stratification and heat storage capacity. The thermal characteristics of three shapes of thermal storage tanks were investigated and analyzed through CFD (Computational Fluid Dynamics) simulation, which were: (a) Cylindrical tank, (b) Circular truncated cone tank and (c) Spherical tank. It was observed that among the three thermal storage tanks, the spherical tank had significant symmetrical temperature stratification, while the truncated circular cone tank had the best temperature stratification and thermal charging efficiency. For the circular truncated cone tank, it was found that when the radius ratio (n) of the water tank was 0.554, it could approach the excellent characteristics of temperature stratification and thermal storage performance. This study could provide a theoretical basis for the structure optimization of the solar energy storage devices, as well as proving to be beneficial to the enhancement of thermal charging efficiency.