利用ＺＹＧＯ光学干涉测量仪，散射积分测量法观测了光学元件表面均方根粗糙度．详细分析了薄Ｓｉ膜对基底均方根粗糙度的影响，由此认为薄膜并不总是复制基底表面的粗糙度，结果出现了薄膜降低表面粗糙度的现象．提出了一定厚度范围的薄Ｓｉ膜的表面粗糙度存在着一个稳定值的新设想． ZYGO optical interferometer was used to measure the root mean square roughness of the surface of the optical element by scattering integral measurement. The effects of thin Si film on root mean square roughness were analyzed in detail. It is concluded that the film does not always replicate the roughness of the surface of the substrate. As a result, the film has the effect of reducing the surface roughness. It is proposed that there is a new idea of ​​the stable value of the surface roughness of the thin Si film with a certain thickness range.