A method of fast design of the low sidelobe omnidirectional waveguide-slot antenna is presented in this paper. Obtain the omnidirectional beam by slotted in both of the top and bottom broad-wall of the waveguide. Considering the mutual coupling, a method of fast design of the slot array of waveguide's wider side based on Elliott equations is applied in the analysis of the Ku-band omnidirectional waveguide-slot antenna, taking the design of the parameters of apertures quickly and optimally. A 2×12 slot array working in the Ku-band is modeled and simulated by CST, an omnidirectional beam is obtained in the E plane, and the sidelobe level in the H plane is suppressed effectively in a certain range of frequency, the results show that the method is feasible.