The invention discloses a composite principle optical fiber sensing system and a sensing method. Continuous light is generated by a laser source, and is divided into local oscillator light and two beams of probe light by a coupler, one beam of probe light passes through one acoustic optical modulator and then is injected from one end of a sensing optical fiber, and the other beam of probe light is converted into a light pulse through the other acoustic optical modulator and injected from the other end of the sensing optical fiber. In front of a photoelectric detector, the local oscillator light, the forward probe light and back scattered light interfere with on another, a beat frequency signal is generated and converted into an electric signal through the photoelectric detector, a difference frequency component of the electric signal is filtered out through a band-pass filtering circuit, beat frequency is lowered through a frequency mixer, the signal is collected and amplified through a collection card, and in an upper computer, amplitude modulation and demodulation of different frequencies are achieved to obtain a forward probe signal and a back scattered light. An improved moving averages algorithm and a peak ratio evening algorithm are used on the back scattered signal to obtain the vibration position. The forward probe signal is analyzed and recognized, the type of vibration is judged, and the false alarm rate is lowered.