Abstract: Based on ray tracing method, the distribution characteristics of evaporation duct and surface duct top electromagnetic shadow zone are analyzed. The results show, that the position of duct top shadow zone will get far away from the position of transmitting antenna with the increase of antenna height, and that will get close to the transmitting antenna with the increase of duct intensity. The area of the top shadow zone mainly changes with the duct top height. Judging from the numbers of trapped rays, surface duct is stronger than evaporation duct in the ability of trapping electromagnetic wave, and both kinds of duct trapping abilities are best at the duct bottom. At last, an approximate calculation formula for the area of the duct top shadow zone is proposed.