Abstract: To improve the performance of the micro-channel condenser outside the tank of domestic air source heat pump water heater, a quasi-steady state model of the household air source heat pump water heater with microchannel condenser was established. The experimental results show that the model error can be controlled within ±9 %. The heat transfer, heat transfer coefficient, and pressure drop of the microchannel condenser were simulated by the model. Results show that when the microchannel condenser has four processes, the pressure drop can be reduced while the heat transfer is guaranteed. The optimal size of microchannel cross section, a key parameter, is between 1.6 and 2.6 mm². The system performance is optimal, and the number of microchannels in the flat tube should be controlled within 16-24. The research results provide reference for the design of micro-channel condenser of household air source heat pump water heater.