Abstract: Response of transient electromagnetism in tunnel whole space was analyzed and ration analytical method to exception response was brought forward.Response of a good conductor was gotten by using the integrated responses of the good conductor in surrounding rock minus the responses of surrounding rock,ration of relative exception response was gotten when the response of a good conductor was divided by response of surrounding rock.Using the ration analytical method of relative exception,effect of transient electromagnetic response was researched due to changing the radius and depth of a good conductor,the distance between a good conductor and the driving source.Radius of a good conductor is bigger and the ratio is higher,and the time of the maximum exception occurring is later.Depth of a good conductor is larger and the ratio is higher,and the time of the maximum exception occurring is later.The distance between a good conductor and the diving source is greater and the ratio is higher,and the time of the maximum exception occurring remains the same.Application of practical engineering shows that the exception in detection data is more easy to distinguish by using the ration analytical method.