Solar water heating system is used to supply hot water all-year-round for a new dormitory building. Flat solar energy collectors are mounted on the roof. The hot water tank and pumps are installed together with the air conditioning equipments in the plant room. Air cooled heat pump is used to provide cooling in summer, and high temperature water from boiler room (in old building) is used as heat source in winter. Usually auxiliary heating is necessary to improve the stability and reliability of solar water heating system. In this case, we take full use of the equipment of air conditioning system instead of electricity as auxiliary heating resources. In this paper, we introduced the design of the solar water heating system and the auxiliary heating method by air conditioning systems. The control strategies to fulfill all the functions and switch between different conditions are also introduced.