A direct-expansion type solar-air source heat pump water heating system was developed.An experimental prototype was investigated under different weather conditions in summer and winter in Nanjing，China.Experimental results showed that the prototype could heat water up to 55℃ efficiently by using different heat sources.When there was a large fluctuation of solar radiation on a sunny day，the collector/evaporator absorbed heat from both solar radiation and air，the prototype operated steadily in the solar-air dual source mode，with a high averaged energy efficiency ratio（EER）of 4.83.Meanwhile，it operated in the air-source mode with an averaged EER of 3.97 in a rainy day.Also，on a cold sunny day in winter，it operated in the solar-source mode with a relatively higher evaporation temperature compared with air-source heat pump，thus avoiding defrosting for the evaporator.