Plane finned tube heat exchanger was widely used in household air conditioner, environmental testing chamber, closed heat source tower, and other heat pump refrigeration systems because of its high heat transfer efficiency, low air resistance, and stable mechanical properties. It was easy to form a frost layer on the surface of flat finned tube heat exchanger working in low temperature and high humidity environment, which greatly reduced heat transfer efficiency and caused waste of energy. The study on the frosting growth on the surface of straight finned tube can provide guidance for the design of plane finned tube heat exchanger. The frosting process of plane finned tube heat exchanger is the result of the interaction of wet air, frosting layer, and heat exchanger structure. In this work, based on the fluid-solid thermal coupling calculation method, considering the changes in frost density and thermal conductivity during the frosting process, a more realistic solution domain boundary was defined, and the frosting process of 3D plane finned tube heat exchanger was numerically simulated. The average error between the simulated frost amount and the experimental value is 4.67%, which is better than the results obtained by previous numerical simulation methods. The frost growth was calculated and compared when the wet air inlet velocity was 1.0, 2.0, 3.0, and 3.7 m/s, and the relative humidity was 60%, 70%, and 80%, respectively. The results showed that the frost thickness decreased along the airflow direction, and the growth rate increased with the increase of velocity and relative humidity. The study on the heat transfer coefficient of air side showed that the higher the relative humidity of air was, the higher the heat transfer coefficient was at the beginning of frosting. In the late frosting period, the greater the relative humidity of the air was, the smaller the heat transfer coefficient was.