To address standoff target tracking using a fixed-wing UAV in unknown background wind, a wind estimation based on Unscented Kalman Filter (UKF) is proposed. In the paper, UAV dynamic model and target state estimation are constructed, and Lyapunov vector field guidance (LVFG) framework is introduced to achieve standoff target tracking. The unknown target state and wind dynamic model have seldom been discussed for standoff tracking in the previous research. Therefore, two typical wind dynamic models with Dryden and Davenport spectrums are constructed for the target in the high altitude or near the ground. Then, the unscented transformation is adopted to estimate the position of UAV under guidance command generated by LVFG and saturation constraint. A wind state estimation method based on UKF is proposed to improve performance of standoff tracking. Simulated and realistic ground vehicle trajectories are used to demonstrate the availability and effectiveness of the proposed method for different wind dynamic models.