The invention discloses an interferometric optical fiber sensor and the digital closed-loop method of controlling action spots of the interferometric optical fiber sensor. A square wave phase modulation signal is introduced between two beams of interference light waves of an interferometer so as to enable the output signal of the interferometer to be changed into a square wave error signal from acosine response signal; the square wave error signal is demodulated by adopting digital correlation detection technology, and the size of low frequency phase drift is obtained; the demodulated low frequency phase drift generates compensating phase shift and sends the compensating phase shift back to a light path for counteracting the low frequency phase drift, and after the process is repeated many times, the interferometer is caused to be at an orthogonal action spot. The invention has the advantages that the low frequency phase drift generated from environmental disturbance is compensated and the orthogonal action spot of the interferometer is stabilized so that a system can work in a most sensitive area always and the stability of the system is enhanced.