The distortion of sine wave can be calculated with the curve-fit method in theory, but sine wave curve-fitting is a four-parameter optimization problem, traditional algorithm in calculating the global optimal solution easily falls into local minima, and thus limits the application of curve-fitting in distortion calculation. For this problem, a three-parameter curve-fit method is introduced and is used in AC distortion measurement. Experimental results show that the method can complete the AC distortion degree measurement and meets the application requirements.