In terms of the problem that SAR(synthetic aperture radar) in UAV(unmanned aerial vehicle) is easy to be attacked by HPM(high-power microwave) weapons in the complex electromagnetic environment of the battlefield, the field-circuit joint simulation of high-power microwave front-gate coupling of UAV′s SAR was carried out. Model of the SAR′s antenna was established in the CST(computer simulation technology) software. Model of antenna was irradiated with a pulse-modulated sinusoidal signal to simulate the HPM signal, and the antenna port coupling voltage waveform was collected. Antenna port coupling voltage signal obtained in the CST software was injected into the RF(radio frequency) front-end circuit model of SAR, which fully simulated the coupling process of high-power microwave signal from field to circuit. Simulation results show that the low noise amplifier in the RF front-end circuit of SAR is the main target of HPM signal damage, and the spike leakage power of the limiter will threaten the low-noise amplifier of the post-stage circuit. Through simulation, the evaluation diagram of the coupling effect of high-power microwave front door of UAV on-board SAR is obtained.