A novel design of high-gain and low-scattering waveguide slot antenna is proposed in this paper. Firstly the scattering pattern of artificial magnetic conductor (AMC) composite surface is estimated by array factor analysis method. The comparison between octagonal ring arrangement and chessboard arrangement proves that the former arrangement has the characteristic of diffuseness-like and expands the bandwidth of radar cross section (RCS) reduction. Secondly, the metal surface of waveguide slot antenna (WSA) is replaced by the octagonal ring arrangement composite surface (ORACS). The gain is improved because of spurious radiation units which are around the slot. At the same time using the phase cancellation principle, a backscatter null achieves RCS reduction in the vertical direction. Experimental results show that the novel antenna after loading with the ORACS, the gain is improved by 5dB; the bandwidth of RCS reduction (reduction greater than 10dB) is 5.24-5.92 GHz.