In order to realize the rapid and accurate evaluation of luminous quality of light guide plate (LGP) and find nonuniform areas to optimize, we propose a new design method based on image processing technology, the evaluation criteria of luminous quality of LGP is deeply researched. The light distribution is represented by gray histogram, the evaluation formula of uniformity is given and the concept of evaluation precision P is presented. Furthermore, the dots position corresponding to discrete grayscale is located with program, the accurate optimization is realized. The method is verified and simulated by taking 184 mm×314 mm×0.55 mm LGP for example. The relation between uniformity and precision is given by analyzing three stage irradiation maps based on gray histogram in the optimization process. The method is compared with conventional evaluation methods. Results show that the gray histogram method can evaluate luminous quality of LGP objectively and comprehensively on the premise of precision and the possible accidental deviation is avoided. The optimization speed and precision are improved. The uniformity of LGP can improve 64% after optimization. The accidental deviation is reduced gradually with TracePro and nine dots in the optimization process. What's more, the results tend to consistent with three evaluation methods.