The present application discloses a fiber distributed acoustic sensing system, including a forward pump source, a wavelength division multiplexer, an active phase-shifted grating array, a backward pump source, and a data demodulation and processing device. The active phase-shifted grating array includes several active phase-shifted gratings engraved on a same fiber, each active phase-shifted grating having a same excitation light wavelength. The active phase-shifted grating array is configured to receive a forward pump pulse and a backward pump light pulse incident from the backward pump source, so that a forward excitation light and a backward excitation light are generated in each active phase-shifted grating, and two adjacent active phase-shifted gratings are enabled to generate excitation light self interference within a same pulse duration. The data demodulation and processing device is configured to perform interference detection on a fiber phase between two adjacent active phase-shifted gratings to detect an acoustic signal acted on the fiber. According to the system in the present application, the passive reflected light interference is replaced with active excitation light interference, thus the excitation light signal has great strength and high stability, and environmental adaptability is high.