To maintain the high efficiency of air source heat pump (ASHP) under frosting conditions, the periodic defrosting is required. The earlier and later defrosting can both cause a decrease in its average coefficient of performance (COP) throughout the frosting-defrosting process. Accurately predicting the frosting rate and using it for defrosting control is a fundamental method to solve this problem. However, there is still lack of a universal model for predicting the frosting rate. To solve this problem, a mathematical model for variable-frequency ASHP was developed firstly, which can simulate its operating parameters under different conditions. Combining with the enthalpy diagram, a calculation method for the initial frosting rate of ASHPs was proposed. Then, an initial frosting rate prediction model suit for all operating conditions was developed utilizing the calculation results. Results indicate that the developed frosting rate prediction model for ASHPs demonstrates well predictive performance. The deviations between the predicted values and actual values are mainly within ±15%. Besides, the initial frosting rate can characterize the overall frosting degree and be used to predict the total frost mass throughout the frosting process. The maximum relative error between the predicted total frosting mass and the experimental results is 9.25%.