Some periodic interference fringes due to medium with inclined angle are found in the echo image when the medium plate is inclined in the penetrating experiment. These interference fringes have serious influence on the imaging result of the interesting target. The study indicates that the interference fringes are caused by the reflected waves, which mainly come from the surface and below layer of the medium plate. To analyze the interference fringe phenomenon, a point source model is constructed in the paper, and the expression of the spacing between the adjacent interference fringes is derived. When the factor about antenna is taken into account, the pyramid horn near-field model is constructed. Based on it, the accurate electromagnetic simulations are performed. The results of the commercial software CST and MATLAB calculation both confirm the analysis about the interference fringes and the relation with the inclined angle of the medium plate. From the expression of the thin medium, the spacing between the adjacent interference fringes is related to incline degree of the medium. Controlling the influence factors of the spacing between fringes can restrain the interference fringe phenomenon in order to improve the target resolving power and imaging quality.