The invention disclosure provides a hot water supply system and method thereof which can control a solar heating unit, a first heat-pump unit, a boiler unit, and an electric heater unit through a control unit to heat water. The control unit operates in a pre-heating period and a heating period after the pre-heating period. In the pre-heating period, the solar heating unit and the first heat-pump unit preheat water. In the heating period, the boiler unit further heats water coming from the solar heating unit and the first heat-pump unit to an exit temperature and stores water in the boiler unit and the electric heater unit so as to provide water to an outflow device. The invention integrates many water heaters in one system, so as to improve the efficiency of generation of hot water and achieve the objectives of energy conservation and carbon reduction.