Because of the inevitable process problems during the manufacturing and installing the folded gratings spectrograph,some sub-images are filted and deformed.To guarantee the wavelength resolution,only one line of pixels is extracted from a sub-image,calibrated,and eventually joined together into spectrum,which leads to low signal to noise ratio(SNR).According to the characteristics of the folded gratings diffraction image,a processing method is put forward.After the image correcting,1-D median filtering and pixels averageing to the folded gratings diffraction image,the spectral SNR is increased by about 6 times,and there is no obvious decrease of the wavelength resolution.The method makes full use of the gratings diffraction image,and is of practical value to improve the detection ability of the folded gratings spectroscope system for weak signals.