Abstract: Abstract: In order to study the distribution and migration properties of refrigerants in a direct-expansion solar-assisted heat pump system, a direct-expansion solar-assisted heat pump water heater (DX-SAHPWH) system, by using R410A as a refrigerant was described, which could supply domestic hot water throughout the whole year. The system mainly consisted of collectors with a total area of 4.2 m2, a compressor with a rated capacity of 0.75 kW, a hot water tank with the volume of 150 L, and an electronic expansion valve. Based on the distributed parameter and homogeneous flow models of solar collector/evaporator and condenser, the lumped parameter models of compressor and electronic expansion valve and the refrigerant charge model, a simulation program of a DX-SAHPWH system using R410A was coded, which was solved by the iterative calculation of the inlet and outlet enthalpy of electronic expansion valve and the refrigerant charge. Then the simulated data and the experimental data of the DX-SAHPWH system using R22 as a refrigerant were compared. Result proved that the mathematical model of the DX-SAHPWH system was reliable. Given the structure and meteorological parameters, initial and final water temperature, for a fixed superheat degree at the outlet of the solar collector/evaporator with a value of 5℃, the effects of various operational and environmental parameters, including water temperature, refrigerant charge, compressor speed, solar radiation, and ambient temperature, on the refrigerant distribution of the system were simulated and analyzed by means of developed model. Simulation results indicate that the refrigerant R410A mostly exists in condensers and solar collectors/evaporators, which accounts for 70% to 90% of refrigerant charge. The change of the operational and environmental parameters affects significantly the content of the refrigerant R410A in the condenser and the solar collector/evaporator, but affects slightly the content of the refrigerant R410A in the compressor and pipes. As expected, a higher refrigerant charge enables the refrigerant mass in the condenser to increase significantly. The refrigerant R410A is transferred from the solar collector/ evaporator to the condenser with increasing water temperature and compressor speed. With the rising solar radiation and ambient temperature, the refrigerant R410A is transferred from the condenser to the solar collector/evaporator. This study is expected to provide the scientific basis for optimizing system performance and reducing the refrigerant charge.