The operation principle of the fiber optic current sensor with an integrated optic modulator is dealt with. The effect of modulation coefficient variation of the integrated optic modulator with temperature on the measuring accuracy is analyzed， and the scale factor errors are simulated. Based on the four-state square wave modulation， a feedback control method of compensating for the modulation channel gain variation resulting from the modulation coefficient is proposed， which effectively reduces the scale factor errors arising from modulator coefficient changes. The effectiveness of the method is shown by the consistency of experimental results with theoretical analysis. Moreover， this feedback control method can reduce the effect of long-term changes with time on the measuring accuracy.