The construction of high-power laser needs a lot of high precision optical windows, and the wavefront detection generally uses phase-shifting interference technology. Due to the multiple reflections of the test light beam in the optical window, there is a fixed pattern phenomenon caused by parasitic interference. Fixed pattern will introduce periodic phase noise into wavefront detection results, which greatly reduces the confidence level of wavefront detection results. To address this problem, this paper presents a method based on wavelet transform to directly filter out the noise from the wavefront detection results according to the characteristics of fixed pattern noise without additional hardware or adjusting the test state. Experimental results demonstrate that this method can effectively filter out the phase noise caused by fixed pattern, and preserve the processing features well.