In view of the problems that the slow processing speed of the traditional phase detection algorithm on the beat signal, a fast signal processing method of frequency-modulated continuous-wave interferometric fiber-optic displacement sensor based on digital signal processing (DSP) technology is proposed. With the DSP technology as the core, fast data transmission is realized through direct memory access (DMA), and the multi-fixed-point peak prediction phase detection algorithm is used to quickly solve the beat signal and optimize the direction. The experimental results show that in the dynamic measurement range of 600 mm, the standard deviation of displacement measurement is less than 5 nm, the linear fitting coefficient is above 0.99997, and the adaptive speed to the measured target reaches 15 mm/s. The proposed method can achieve higher measurement accuracy and faster dynamic measurement speed.