In view of the difficulty in detecting the high reflective surface defects in the bottom of a large metal barrel, an endoscopic illumination scheme is proposed. Comprehensively considering the structural and surface characteristics of the metal barrel, a simulation study on the non-forward illumination scheme of the barrel bottom is conducted based on the bidirectional scattering distribution function (BSDF). The results show that the illuminance at the bottom of the barrel presents a good uniformity under theoretical conditions. The corresponding experimental verification results indicate that the actual illumination conditions are basically the same as the simulation results. The proposed scheme can obtain better image quality than the general illumination scheme.