In this work, we establish a novel numerical model of total ionizing dose effect and use it to simulate the radiation degradation of Si n-metal-oxide-semiconductor field effect transistor (NMOSFET) under different bias voltages. The model is based on the capture/emission process of traps, and is used to simulate the transient characteristics of semiconductor devices under total ionizing dose effect. In the simulation, the changes of trapped holes in Si/SiO₂ interface and gate oxide layer are extracted, and it is found that the number of trapped holes at different positions tends to be saturated with the increase of the total dose. When the radiation bias voltage is positive, the degradation amplitude of the threshold voltage is significantly higher than that when the radiation bias voltage is negative. Whether the gate is applied with positive bias or negative bias during the radiation, the degradation amplitude of the threshold voltage shows a trend of first increasing and then decreasing with the increase of the absolute value of radiation bias voltage. Radiation bias voltage also has a certain effect on the annealing effect after radiation. If a gate bias voltage is applied to the device during the annealing, the electrical characteristics recovery amplitude of the device is lower than that under zero bias voltage.