The structure of modular air-cooled cold water heater unit is four-way valve which is connected with compressor, heat exchanger and fin heat exchanger respectively, and economizer is connected with heat exchanger and expansion valve respectively. The expansion valve is connected with the finned heat exchanger and connected with the gas-liquid separator. The gas-liquid separator is also connected with the compressor. Another connecting pipe is led out between the expansion valve and the finned heat exchanger and connected with the gas-liquid separator through the left solenoid valve. A balance tank is installed in the middle of the connecting pipe between the heat exchanger and the economizer, A hot gas pipe is drawn out between the exhaust port of the compressor and the connecting pipe of the four-way valve, which is connected with the gas-liquid separator through the solenoid valve. The modular air-cooled chiller and hot water unit can solve the problem of liquid return when heating at low temperature and can't exert the heating capacity of the system due to the lack of refrigerant. Compared with the existing products, the refrigerant charging capacity is larger, the heating capacity is larger, the efficiency is higher, the operation cost is reduced, there is no liquid return, and the stability of the unit is improved.