Based on the known light intensity distribution of a light source and indoor spatial structure, this study established a mathematical model and adjusted the relevant parameters through experiments with respect to a hypothesis about the bidirectional reflectance distribution function (BRDF) of the spatial surface. Further, the application of this mathematical model in different types of spatial models results in an average illuminance error of less than 3%. Moreover, the plane illumination curve distribution is basically similar to those obtained using the existing methods. The working-surface related parameter values are compared with the DIALux simulation results. This study also improves the BRDF model of a wall surface in terms of the application of the uneven light energy distribution on the reflective part. The proposed method improves the illuminance calculation accuracy in a circular bottom space or a space with its bottom not equal to height where a direct calculation is not available.