The infrared reflection characteristics of the wall are characterized and solved by the bidirectional reflectance distribution function (BRDF). BRDF measurement currently has two problems to be addressed: it requires much experimental data and accuracy is not high enough. By constructing the reflection characteristic test platform of the wall target, an MR170 Fourier infrared spectroradiometer was used to obtain the target radiance at the incident angle and each reflection angle in the 2-15 μm band. For the stealth target, the RBF network was used to fit the radiance at the bands of 3-5 μm and 8-14 μm to eliminate atmospheric interference. Then, the BRDF values of the stealth targets in the above two bands were obtained. To improve the accuracy of the BRDF model, an improved whale optimization algorithm (IWOA) was proposed to invert BRDF model parameters, and a reflectivity-solving method based on BRDF was designed. The IWOA has a good effect on the parameter inversion of the BRDF calculation model. According to the reflection method and applying the obtained BRDF data, the reflectance 0.5496 and the relative error 6.17% are obtained, which meet the engineering requirements.