Abstract: In high power laser facilities, the GRMS is a key specification to evaluate the low frequency wavefront distortion. While the deformable mirror is widely used to correct the distortion, the raw GRMS specification should include the effect of deformable mirror. The article presents the method to decompose the influence of deformable mirror and the partial coherent characteristic of corrected wavefront. The relation between the GRMS and the size of laser focus of corrected wavefront is verified. Based on the research, the procedure of setting up the GRMS specification of the optics is established.