Applications of air-source heat pump are steadily increasing in South China especially in Yangtze River region. Owing to low temperature and high humidity in these areas, frost formation is liable to occur on the outdoor heat exchanger, thus resulting in reduction in performance of the heat pump and even failure of some components such as the compressor and the motor. In order to improve performance and reliability of the air-source heat pump operating in cold and humid climate, rational defrosting control is important, and this should be based on understanding of characteristics of the heat pump operating under frosting/defrosting conditions. In this paper, operating behavior of an air-source heat pump under frosting/defrosting conditions is investigated experimentally. Recorded are dynamic changes in parameters of the heat pump under frosting/defrosting conditions, including suction and discharge pressure of the compressor, temperature of the fin surface, heating capacity, power consumption, electric current of the fan and thickness of the frost layer. Effect of frost formation on performance and reliability of the heat pump is analyzed. It is shown that there is not a great change in operating state as well as performance of the air-source heat pump until the frost layer is accumulated to some extent, and that performance can be improved by taking proper defrosting measurements. Furthermore, rapid changes in suction pressure and discharge pressure under defrosting conditions may result in failure of the compressor and the motor, and therefore, reducing changes in pressure especially of suction is important for improving reliability of the heat pump.