Aiming at the problem of emissivity solution using the reflection method, an indirect solution method of emissivity based on bidirectional reflectance distribution function (BRDF) is proposed. A reflection brightness measurement system is designed, and the wall reflection brightness data at multiple angles are measured by the MR170 radiometer. The reflection brightness data in the 3－5 m and 8－14 m bands are regressed by the gated recurrent unit (GRU) network to eliminate the interference of atmospheric absorption. In order to accurately invert the BRDF model parameters, the algorithm is improved by adding three strategies: chaotic mapping initialization, Cauchy mutation and variable spiral search on the basis of the dung beetle optimization (DBO) algorithm. After calculating the BRDF values at different angles, the parameter values of the five-parameter function model are inverted by the improved algorithm, and then the infrared radiation distribution in the entire space is obtained. Compared with other algorithms, the convergence speed of the improved algorithm is greatly improved, the final emissivity is 0.4236, and the relative error between the theoretical test sample and the algorithm is 11.75%. This study can provide a reference for indirectly solving the emissivity problem.