In this paper, 1991 March 24 CME's structure is analyzed by determining the moment of sudden change in galactic cosmic ray intensites using wavelet method based on the sudden change in galactic cosmic ray intensities when a CME just arrives at the magnetosphere. The results show that the thickness of the CME_driven sho ck is about 02228AU,while the space scale from the front side of the CME to th e most intense magnetic structure is about 0325AU.