Abstract: Mainly focusing on the phenomenon that the phase will delay by using the FIR digital filter in signal processing, starting from the phase characteristic of the FIR digital filter, this paper analyzes and deduces the reasons of this phenomenon and obtains the relationships of the order and the phase delay. This paper adopts the method based on waveform matching to extend data of the original signal, and then filters the signal by FIR filtering. The Matlab simulation shows that the relationships between the order and the phase delay completely consist with the theoretical deduction. The phenomenon is eliminated completely. The results of Matlab simulation show that filtering the signal using the method proposed in this paper can completely eliminate the phase delay and obtain the wave phase consisting with the initial signal. Through analyzing the actual signal with simulink, the proposed method can effectively eliminate the phenomenon of phase delay caused by FIR digital filter.