A Narrow Bandpass Filter (NBF) with finite impulse response for denoising the original signal is designed based on the characteristics of the frequency spectrum and the autocorrelation algorithm. Frequency spectrum correction technologies were used in beam expanded differential Laser Doppler Velocimeter (LDV) to process the signal simultaneously. Experimental results indicated that the random and high-frequency noise of the signal were mainly removed by the NBF. The frequency spectrum of the Doppler signal after the NBF was acquired by windowed Fast Fourier Transform (FFT). The measurement accuracy of frequency was enhanced by the frequency spectrum correction algorithm. Therefore, the signal processing system, including the NBF and spectrum correction, is suitable for the beam expanded differential LDV.