The effects of flash tank economizer on performance of air source screw heat pump are studied. The effect of economizer vapor injection pressure on unit performance is theoretically analyzed. The study shows that the lower economizer vapor injection pressure is

the higher heating capacity and compressor power of screw heat pump with flash type economizer are. The COP of heat pump first increased and reduced again with the increase of the economizer vapor injection pressure. There is an optimized point. The experiment was carried out to investigate the effects of flash tank economizer on performance of heat pump at near optimum economizer vapor injection pressure. The simulation results are in good agreement with those of the experiment.