The low energy consumption development of smart grid requires that the power supply equipment distortion in the power system should be within a certain range. Aiming at the problem that it is difficult to calibrate the distortion degree of the output current of AC power supply, the influencing factors of the distortion are analyzed by theoretical deduction. A novel calibration method for AC power supply using a shunt is proposed, which can significantly reduce the calibration uncertainty. The experimental results show that the AC distortion degree of shunt calibration method is suitable for a larger current output range, which makes up for the limited current range of some power analyzers and reduces the uncertainty of distortion calibration. Monte Carlo method is used to simulate and calculate the distortion caused by the shunt method due to the variation of resistance value, and the uncertainty theory is used to verify the obvious advantages of the shunt method in the calibration of AC power supply current distortion. The uncertainty is reduced from 0.996% to 0.2%. This method can be used as an optimization calibration method of power distortion degree for smart grid service, which can help smart grid reduce noise interference and efficiently use energy.