High power microwave (HPM) can disrupt the normal work of electronic systems through the effect of HPM on semiconductor devices. In this paper, the physical process and the physical model of the characteristic degradation of n-metal-oxide-semiconductor field-effect transistor (nMOSFET) induced by HPM are introduced. In device simulation results, the output characteristic curve of nMOSFET shows that HPM can induce the degradation of the characteristics of device, including the forward drift of threshold voltage, and the reduction of saturation current and transconductance. Based on the process and the model introduced in this paper, the voltage pulse generated by HPM makes nMOSFET be in depletion status and hot carrier increase; then the effect of hot carrier results in the characteristic degradation of device. The experimental result of MOS injected HPM shows the changes of output characteristics and model parameters are consistent with the device simulation results, proving that the physical process and the physical model introduced in the paper are correct.