Abstract: In order to make sure of adequate lighting for the driver in the fog, snow, rain or dust and other terrible weather conditions, and prevent the opposite driver from glaring, a front fog lamp system with a cut-off line was designed. Based on the theory of non-imaging optics, an optimal illumination design method was adopted and numerical coordinates were used to solve the coordinates of each point on the free-form surface mirror (FFR). To achieve light distribution requirement, the fundamental surfaces were divided legitimately and the spread angle of surfaces were adjusted. Through tracing the light with Monte Carlo simulation method, the simulation result shows that the final effect of light distribution fully fulfills the photometric characteristics of power-driven vehicle front fog lamps regulations(GB4660-2016), the optical system has a clear cut-off line, the volume is small, the edge area also has higher light energy, and the light utilization rate reaches 52.7%.