In the measurement system of the backscattered light of a gravitational wave telescope, the beam splitter plays a vital role. Its core function is to efficiently separate the incident light from the backscattered light. The intensity of the light beam output by the beam splitter is directly related to the accuracy of the final measurement results. As the structural basis of the beam splitter, the scattering effect caused by the transparent substrate will further affect the accuracy of the overall measurement system. The scattered light of the transparent substrate mainly comes from the surface scattering induced by surface roughness and the volume scattering caused by the inhomogeneity of the refractive index inside the substrate. When the light beam impinges on the surface of the transparent substrate, these two scattering mechanisms are coupled with each other, making it extremely difficult to measure either of them separately. In response to this challenge, this study has set up an experimental optical path based on the theory of polarization scattering.This study analyzed various scattered light testing methods. Based on this, an experimental optical path (Fig.6) was established using the theory of polarization scattering. The volume scattering of transparent substrates made from different base materials, the volume scattering of substrates with the same base material but different thicknesses, and the scattering introduced by sample pieces with identical base material, same thickness but different surface roughness were compared. A transparent substrate with a thickness of 3 mm and surface roughness of 3 nm was selected as the experimental sample. This sample was used to measure surface and volume scattering under different incident and scattering angles (Tab.5), thereby determining the distribution of surface and volume scattering in transparent substrates across various incident and scattering angles. Additionally, a transparent substrate with 0.3 nm roughness and 3 mm thickness was used to validate the accuracy of the testing system.According to the basic principles of the polarization scattering theory, the polarization directions of the surface scattering and volume scattering under different incident angles and scattering angles are obtained. By using the principle of polarization separation, either the surface scattering or the volume scattering can be completely eliminated to achieve the purpose of separating the surface scattering from the volume scattering. An experimental optical path based on the polarization scattering theory was set up. A sapphire substrate with a thickness of 3 mm and a roughness of 3 nm was used as a sample (Tab.4), and a detector with adjustable gain was selected to measure the surface scattering and volume scattering under different incident angles and scattering angles (Tab.5). The distributions of the surface scattering and volume scattering of the transparent substrate under different incident angles and scattering angles were determined.An experimental optical path was set up based on the principle of polarization scattering. A sapphire substrate with a thickness of 3 mm and a roughness of 3 nm, which has the same surface scattering intensity and volume scattering intensity, was selected as the experimental sample. Meanwhile, a detector with adjustable gain was chosen to measure the surface scattering and volume scattering at different incident angles and scattering angles. Then, a sapphire sample with a thickness of 3 mm and a surface roughness of 0.3 nm was used to verify the accuracy of the experimental results.The experimental results show that this method can successfully separate and independently measure the surface scattering and volume scattering. This solution provides a testing basis for the subsequent back - scattered light measurement system of high - sensitivity gravitational wave telescopes to screen materials with low volume scattering at a roughness of 3 - 8 nm.