In order to meet the requirements of precision measurement of micro-amplitude DC signals, a fully digital magnetic modulator for accurately measuring micro-ampere DC is designed. It converts the excitation terminal and the output terminal voltage into digital signals, and then performs Fourier analysis, vector method zero adjustment and proportional coefficient correction to obtain DC measurement values in real time. The measurement results show that the accuracy can reach 0.5% and the linear error is better than 2×10-4 when the measurement range is 5μA-1000μA, and the measurement error caused by the excitation source frequency and amplitude variation is effectively suppressed.