Application of light scattering measurement in the inspection of fused silica substrates for laser-gyro application was studied. It was found that collecting objective is more propitious to eliminate volume scattering than integrating sphere and more appropriate to the measurement of scattering light for directly detecting supersmooth surfaces of fused silica substrates. Collecting objective(N.A. is 0.4) was designed by ZEMAX and volume scattering from points whose axile distances are more than 1.5 millimeters was eliminated completely. To solve the drift problem of photomultiplier tube, specific structure of collecting system was designed, so that scattering light and reflecting light can strike on the same photmulitplier tube on separate time. It was found that the drift problem of photomultipolier tube can be solved with this setting, so that the stability of the scattering measurement system over long time span can be improved greatly and its practicability in engineering is ensured.