Voronoi diagram is often used for UCAV path planning in SEAD mission. General Voronoi diagram is constructed by connecting the circum-centers of Delaunay triangles, while every vertex of Voronoi diagram is considered as a kind of enemy air defense. But there are a lot of enemy air defenses in the battlefield, and every enemy weapon has its action area, so that the perpendicular bisector of Delaunay among different threats which is the edge of Voronoi diagram is no longer the safe path of UCAV. Firstly, the current study proposed a novel method to modify the general Voronoi diagram so that the generated improved Voronoi diagram could be applicable for the UCAV path planning, and the criteria of constructing the improved Voronoi diagram were presented. Secondly, based on “Delaunay polygon”, it presented all the possible conditions which the improved Voronoi diagram could be constructed. Finally, the simulation demonstrates that the planned path based on the improved Voronoi diagram can increase the survival probability of UCAVs in SEAD mission obviously.