Abstract: A time domain model for complex envelope simulation of multipath effect is presented based on multi-reflect-point target model. The model was used to calculate return waves of flat and fluctuant surfaces of land or sea. The 3D surface model of land and sea was built by the random fractal interpolation method with measured data. Signals from various roughness surfaces were computed. Simulation of low altitude target track under multipath effect environment was conducted with the active seeker model. The results indicated that multipath effect can bring large error for low altitude target tracking.