The invention relates to the field of electrical engineering, and particularly to a device and a method for modulation phase disturbance compensation of an all-fiber current transformer. A photosensitive sensor in a light strength detecting unit detects light source of a detecting light source of a phase modulator in the all-fiber current transformer. A detected signal is amplified through a signal amplifying triode of a signal amplifying unit. The detected signal is converted to a digital signal through an analog-to-digital conversion unit, and then the digital signal is transmitted to a signal processing unit for processing. According to the device and the method, a light source phase disturbance in the all-fiber current transformer is modulated through a weighted minimum mean square error criterion. The device performs real-time monitoring on an error in a phase modulating system. Through introducing a weighting function factor and continuously iterating the weighting function, an expected modulation phase is obtained for solving the modulation angle of the phase modulator in the all-fiber current transformer, thereby reducing a minimal offset error of a coupler phase caused by a non-matching property and asymmetry of the fiber in actual manufacture, and improving performance of the all-fiber current transformer.