Abstract: An indirect illumination pattern with flat LED array and diffuse reflection freeform surface was proposed. Based on the Lambertian characteristic of LEDs and ideal diffuse surface, a mathematical model was constructed. A set of differential equations were established according to the expected energy distribution on the target plane. Through solving these equations numerically, the outline of the diffuse surface was obtained. By importing the entity model of the diffuse reflection illumination system into optical software to perform non-sequential ray tracing, the simulation result shows that the irradiance uniformity of the detection area is above 90% and demonstrates that the proposed method is correct and effective.