One of the major problems of designing an illumination optical system is the repeatedly time-consuming ray-tracing process while optimizing the illumination optical system parameters. This study proposes a "one-time ray-tracing method" for the optimization of illumination systems. As a demonstration, this study designs a planar lightguide backlight with uniformly irradiance at the light-exiting surface using the proposed one-time ray-tracing method. In this method, the irradiance distributions of the target region are related to the radiant intensity distribution of the side planar light sources. Using only one-time ray tracing and the optimization of the light source radiant intensity, the proposed method can quickly find a design of the planar lightguide backlight for providing an uniformly irradiance distribution at target illumination region. That is, with only one-time ray tracing and the optimization technique, the CV(RMSR) (coefficient of variation of root mean square error), of the target region irradiance distribution was decreased to 0.06 from 0.12. The proposed method holds great potential to the designing illumination systems, since it can be applied to design not only the planar lightguide but also other optical illumination systems.