Based on the production kinetics of oxide-trapped charge and interface-trapped charge and the microscopic mechanism of radiation damage, a model of post-irradiation threshold voltage drift due to oxide trap and interface trap as a function of radiation dose is proposed. This model predicts that the post-irradiation threshold voltage drift due to oxide trap and interface trap would be linear in dose at low dose levels. At high dose levels, the post-irradiation threshold voltage drift due to oxide trap tend to be saturated, its peak value has no correlation with radiation dose, and the post-irradiation threshold voltage drift due to interface trap has an exponential relationship with radiation dose. In addition, the model indicates that the oxide-trapped charge and the interface-trapped charge start a saturation phenomenon at different radiation doses, and the saturation phenomenon of oxide-trapped charge appears earlier than interface-trapped charge. Finally, the experimental results accord well with the model. This model provides a more accurate prediction for radiation damage in metal-oxide-semiconductor field effect transistor.