Abstract: The accuracy of the secondary electron yield(SEY) model has a significant impact on the exact prediction of the multipcator thresholds. In view of the shortcomings of the existing SEY(secondary electron yield) phenomenological model, a new combined semi-empirical model for calculating the SEY is established by using the idea of piecewise function. The model utilizes the corrected Vaughan model to calculate the SEY when the incident energy E_p < E_pm (E_pm refers to the incident electron energy corresponding to the maximum value of secondary electron emission yield), and a semi-empirical model is establishedto calculate the SEY when the incident energy E_p≥E_pm. The combined semi-empirical model is validated by experimental results of the SEY for molybdenum and silver. The simulation results show that the combined semi-empirical model can accurately characterize the SEY before and after sputtering cleaning as well as different incident angles. The average and mean square deviation of relative errors are significantly lower than those of the Vaughan model and semi-empirical model. Moreover, the model has good stability under different incident electron angles, which can greatly improve the fitting accuracy between the simulation results of secondary electron emission model and test data. This model is expected to be applied to simulate the multipactor threshold of high-power microwave components in space.