In order to address the problem that uncertain environments may cause uncertain disturbances to the motion planning of unmanned surface vehicles (USV) due to the effects of wind and current, we propose the theoretical trajectory model (TM) and the algorithm of the motion planning bound, which will provide the reachable areas for USV navigation by computing position error margin. Firstly, the mathematics model of USV motion in uncertain environments is built, in which the effects of wind and current are uncertain; how the trajectory could be influenced by uncertain environments is analyzed. Secondly, the TM and an algorithm for computing reachable areas are constructed based on the above model and analysis. Thirdly, the different TMs are discussed and built according to the different situations. Finally, in uncertain environments (e.g. fluctuating wind environments), our method can help to achieve safe navigation for USVs, the experiments are carried out to evaluate the effectiveness of our proposed model and algorithm.