The principle of regulation of wattless power in Magnetic-Valve Controllable Reactor is introduced in this paper. The characteristic of magnetic path and the character of DC magnetic bias are explained. The output current with DC magnetic bias is analyzed by using the method of conversion in scale. Electromagnetic field software of ANSYS is applied for calculating the output current with different DC magnetic bias at the same time. In order to find out the relationship between magnetic saturation value and harmonic, the method of data interpolation, curve matching and FFT by Matlab is applied. The result provides reference value for practical engineering application.