In this study, a phase-change heat-storage water tank with a flat-plate storage plate was designed with phase-change materials. A heat-storage water tank model was constructed using the same staggered arrangement. Experiments and simulations were used to investigate the heat storage and discharge performances of a flat-plate-filled phase-change heat-storage water tank and the influence of the discharge flow rate on the heat release performance of the change materials. The role of different discharge flow rates in improving the heat release performance of phase change materials is discussed. The results show that filling a 98 L water tank with 12.15 L of phase change materials in a flat plate manner increased the heat storage capacity of the water tank by 17.91% while improving the stability of the system operation. By analyzing the flow rates under different heat release modes, it was found that at a discharge flow rate of 100-175 L/h, the heat release ratio of the phase change materials was maintained above 88%. The hot water supply volume reached 135 L. However, the heat transfer between the fluid and the phase change materials was no longer timely when the flow rate increased. The heat release ratio of the phase change materials gradually decreased to 81.5%, and the hot-water supply volume decreased to 125 L. To improve the heat discharge performance of phase change materials in a heat-storage water tank, it is necessary to ensure sufficient external heat transfer conditions and fundamentally improve the heat exchange capacity of the phase change materials.