This paper presents a fidelity SAR simulator for forest region considering raw echoes generation and image focusing process with various kinds of uncertain factors including trajectory deviations, attitude dithering, calibration error, noises. Moreover, a rapid method is also developed to reduce the time cost of raw signal generation for three-dimensional forest canopies. The simulator is verified by comparing with E-SAR data and PolSARproSIM data via forest structure inversion. The results indicate that the simulated images by the proposed simulator have the similar features with E-SAR images and the efficiency is significantly improved compared with PolSARproSIM. Further applications of the simulator show its abilities on large area forest simulation with uncertain factors of SAR system.