On the basis of the principle of Ground Penetrating Radar (GPR) method and geophysical characteristics, this paper discusses in detail detection method of civil air defense distinguished by GPR under the complex geological condition through using the analysis and application in the survey of underground civil air defense as an example. Three dimensional image of the defense clearly reflects its underground structure. Test result has the greatly high detection precision. This example illustrates the effectiveness and practicability of GPR in the respect of detection of the civil air defense and also accumulates experiences for the application of GPR in urban geological survey.