Abstract: In this paper, the effect of the cavity depth on the radiation of the slot antenna is analyzed with the innovative waveguide reflection model. With the help of the simulation results, we make sure that when the cavity depth equals to one quarter guide wavelength, the cavity will double the input resistance of the slot antenna, but has small effect on the input reactance. According to the simulation result, an effective input resistance circuit model of the cavity-backed slot antenna(CBSA)is built. The effect of the feed position and slot length on the resonant frequency is also analyzed, which is the theoretical basis for the design of multi-frequency slot antenna. Based on the simulation tool, two kinds of CBSA working at Wi-Fi band are designed. One kind works at 2.4 GHz for outdoor application, another works at 2.4 GHz and 5.2 GHz. In the end, the S₁₁ coefficient of the antenna is tested, the measurement results agree well with the simulation results.