The ground penetrating radar has such merits as high-resolution, nondestructive operation, rapidness and intuitiveness, and these merits play an important role in the quality of grouting testing of prestressed duct. Using the generalized completely Matched Layer (GPML) as absorbing boundary and based on the Maxwell Finite Difference Time Domain methods (FDTD) and Yee grid, the authors made two-dimensional GPR simulation of the water and gas saturated grouting holes with the same depth and different shapes in the steel strand and, in combination with engineering window verification, summarized the characteristics and regularity of duct grouting defects of radar image. The method provides essential foundation for the grouting fullness degree of corrugated pipe tested by ground penetrating radar.