Radar cross section (RCS) quantity of stealth Unmanned Aerial Vehicle (UAV) varies with angles of UAV body. Multi-radar network is a grave threat to stealth UAV as it is able to detect aircraft in different angles, which increases the risks of stealth UAV. Hence it's necessary to develop the trajectory planning algorithms for stealth UAV flying in multi-radar environment. In this essay, we introduce a fast trajectory planning algorithm to deal with such situation. A special A∗ algorithm considering UAV flight limitation and influence of all radars in the multi-radar network is designed. At last, a simulation experiment is implied to verify the efficiency of our algorithm.