Parasitic reflection is one of the most important fact impacts the laser′s near-field quality. Based on Bespalov-Talanov theory, the small-scale self-focusing caused by parasitic reflection and the parameter′s contribution to the growth of ripple in wedged optical device of high power system are discussed. By analyzing the interference field caused by the main beam and the reflected beam, the expression of small-scale modulated gain spectrum in no-linear media is found. The laws that the maxium gain of small-scale modulation grows with the window′s wedged angle of Nd-glass slab amplifier when the media gain or propagation distance changed are concluded. The influence of the length and wedge angle of amplifier window on the gain of perturbation is analyzed, additionally variation relation between the gain of perturbation and the width of the pulse with fixed window is derived.