Abstract: This paper studies the high resistance surface EBG, whose mushroom-like two dimensional construction is compact, periodic and the element size is much shorter than the operating wavelength. This kind of EBG is suitable for array antennas, its surface wave depression character can improve antenna performance. The original antenna structure parameters of the mushroom-like EBG were selected for 12GHz and validated with coaxial probe mode. Then, loading the mushroom-like EBG on the waveguide slot array antenna and optimizing the design with HFSS simulation, the sidelobe and backlobe of pattern was decreased, among the rest, the second sidelobe and backlobe depression is greater than 2dB, furthermore,forward radiation has some increase.