Abstract: The helical coil phase change energy storage tanks exhibit high heat transfer efficiency and have wide spread application. The impact of geometric parameters of the helical coil—diameter ratio and compression ratio on the heat transfer characteristics within a vertical phase change thermal storage unit was investigated. First, the variation trend of liquid phase fraction and average temperature of the phase change material (PCM) during thermal storage process was analyzed for energy storage tanks with diameter ratios ranging from 0.5 to 0.8. The total thermal storage capacity, melting time and average thermal storage rate of the PCM were compared and analyzed. Second, using melting time and total thermal storage capacity as evaluation indexes, entropy weight method and technique for order preference by similarity to an ideal solution (TOPSIS) were employed to assess the effect of diameter ratio on thermal storage performance. Finally, the evolution of solid-liquid interface and vorticity of the PCM was analyzed to reveal the intrinsic mechanism of heat transfer enhancement via gravity-driven natural convection. Additionally, focusing on the energy storage tank with a diameter ratio of 0.65, the impact of variable-pitch helical coil structure on heat transfer performance was studied. Results show that an appropriate compression ratio can enhance thermal storage performance, and when the compression ratio exceeds a reasonable range, the thermal storage performance of energy storage tanks will decline. According to this research, the optimal compression ratio for the helical coil is determined to be 2, as compared with the standard energy storage tank, achieving a 42.47% reduction in complete melting time for the PCM and a 66.66% increase in average thermal storage rate. The energy storage tank with a diameter ratio of 0.65 has the best thermal storage performance.