For fast design of millimeter-wave non-resonant slotted waveguide antennas (SWAs) with more accuracy, an improved analytical method is presented by introducing a modified factor into the traditional Stevensons formula. The modified factor can greatly improve analytical calculation accuracy of slot displacement of longitudinal shunt slots cut in the broad face of a waveguide. Generally, the traditional Stevensons formula is valid for only narrow slots of a length-to-width ratio of 10∶1. However, the proposed method allows for fast design of millimeter-wave SWAs of very low-sidelobe levels and wide slots of an approximately 4∶1 length-to-width ratio. A theoretical explanation of the modification was provided. With the improved method, a prototype of non-resonant SWAs operating in 60 GHz band was designed and fabricated. The measured gain and sidelobe level are 14.8 dB and -25 dB. Good agreement between the simulation results and measured data indicate the effectiveness of this improved method.