Heat pump is an important means of energy saving, with which energy can be transferred from the low-temperature heat source to high temperature heat source. Compared with other heat systems, it offers an attractive option for heating and cooling residential and commercial buildings owing to its higher energy efficiency. The coefficient of performance (COP) of conventional heat pump is generally from 3 to 5 home and abroad. This paper presents an optimization method to design a heat pump water heater system by adding a heat recovery unit of proper capacity to recover the heat source in the sewage. For optimization, the relationship between the evaporation temperature, condensation temperature and system energy efficiency has been analyzed. It is proved in the paper that as the heat recovery unit has larger capacity of heat exchanging, the whole system has a higher coefficient of performance (COP). Experimental results show that the system's COP can achieve 6, which proves that the optimization design makes the system more efficient and productive.