Abstract: Radars always display the anomalous performance characteristics such as Over The Horizon (OTH) detection capability when they are operated in marine environments because they are influenced by evaporation duct which is a kind of familiar natural phenomenon. To evaluate accurately the ability of radar in evaporation duct environments, a method is proposed which is combined the electromagnetic waves propagation numerical equation with the theories of radar receiver. The data extracted from the experiment of Dalian sea region in October of 2003 is analyzed using this method, and the results of this method are quiet agreement with the actual radar detection.