Because the solar heat pump water heater is working all the year round with variable environment condition, the thermal coefficient is lower in winter and the discharge temperature of compressor is too high in summer and winter. The operating parameters of the system are closely linked to the discharge temperature of compressor and COP. This paper researches the extent influence on discharge temperature of compressor and the COP of system from evaporation temperature, condensing temperature, sub-cooling degree and superheat temperature basing on the running range condition of normal temperature heat pump hot water system. A great number of different combinations of operating parameters will appear on the heat pump theoretical cycle, then the COP of such heat pump system and discharge temperature of compressor will be analog computed. The result shows that the discharge temperature of compressor has positive proportional approximately relationship with the suction superheating temperature of compressor and condensing temperature; the lower evaporating temperature is the main reason for too high discharge temperature and low COP in winter. Based on the above theoretical analytical results, we designed a new solar heat pump hot water system to solve the problems of heat pump system and did the experiment to prove it. In summer, the discharge temperature of compressor can be reduced about 10℃; In winter, the COP can be increased by20~30% and the discharge temperature of compressor can be decreased by 10.7%.