The core algorithm to prevent unmanned aircraft system（UAS）from flying out of the designated airspace or flying into the restricted airspace is the goe-fence violation detection algorithm of UAS. In order to improve the accuracy of cross-border detection of UAS geo-fence and ensure the safety of low-altitude UAS flight, this paper improves the ray cast algorithm by adding buffer distance to solve three special cases that cannot be solved by the traditional ray cast algorithm, and it is verified by simulation under Matlab environment, and the detection function of crossing the boundary is realized. Three factors that affect the running time of the improved algorithm are found out, and in-depth analysis and discussion are carried out. Simulation results show that the proposed goe-fence violation detection algorithm based on improved ray cast algorithm improves the accuracy of UAS detection without affecting the running time.