The frequency characteristic of the admittance of radiating slot and the waveguide wavelength of radiating waveguide is the main factor that influences the bandwidth of slot array antenna. Based on the idea of improving the frequency characteristic of the two parameters above, a novel method of improving the bandwidth of slot array antenna is presented in this paper, which uses a ridged waveguide instead of a rectangular waveguide as the radiating waveguide. The simulation analysis has indicated that the bandwidth of ridged waveguide slot array has been greatly improved, and it is consistent with the result of thoery. The result of this paper brings forword a new idea for improving the bandwidth of slot array antenna.