A two path and self-correlation system is designed to measure reflectance and transmittance. In order to verify and modify the stability of this system, repeatability of reflectance and transmittance for single-point is tested and analyzed by using of three different laser sources, and it concludes that the power and polarization stability of the system does little reaction with system repeatability. Stability is enhanced based on experimental analysis, what’s more, system accuracy is calibrated. The result turns out that the uncertainty is stabilized at 0.04%, which fits well with the accuracy requirement of uniformity measurement on large-aperture optics with low transmittance or reflectance, and concrete parameter selections are provided to the instrument design of this kind.