In the process of fabrication and installation, defects will inevitably be introduced into the mirror used in laser gyro, which will modulate the incident light and induce light scattering. In order to study the variation law of scattering field when scratch defect size changes, the scattering model of single scratch defect with rectangular cross-section on the surface of high reflection mirror is established by using finite element method and multi physical field simulation software. Analyze the spatial distribution of laser scattering field by changing the width and depth of the defect; The integrated scattering measurement system is built to detect the defects with different depth and width, and compare them with the simulation results. Experimental results show that the change trend of scattering field of defects with different sizes is basically consistent with the change trend of simulation results, which provide theoretical basis and reference for the detection of surface defects of high-reflection mirrors.