Air source heat pump (ASHP) has high efficiency when space heating. However, under humid and low temperature conditions, outdoor heat exchanger will be covered by frost. So high effective defrosting is a challenge for ASHP. In this investigation, a review is carried on defrosting technologies used in ASHP. Electric heater defrosting (EHD), reverse cycle defrosting (RCD), hot gas bypass defrosting (HGBD) and thermal energy storage defrosting (TESD) are reviewed, and the efficiencies of these four defrosting are compared. Electric heater defrosting converts power to thermal energy to melt frost directly. The efficiency of electric heat defrosting is the smallest, which is smaller than 1 because of the big heat loss during defrosting. Hot gas bypass defrosting converts power to thermal energy through compressor to defrost. Its efficiency is from 1 to 2.2. Although reverse cycle has the highest defrosting efficiency 3.96, the indoor air temperature will decrease while defrosting. The efficiency of phase change energy storage defrosting is 3.21. The efficiency of phase change energy storage defrosting is smaller than reverse cycle defrosting, but it will not result in indoor air temperature decreasing and shorten the time of defrosting.