Abstract: On the basis of studying the general method of sine fitting, a new approach of testing the A/D dynamic characteristic with the harmonic distortion existing is put forward. This approach uses the idea of circularly approximating the minimum of the fitting error to get the function of sine fitting. At the same time, dichotomy is used to improve the algorithmic velocity. It not only gets rid of the influence of harmonic distortion but also avoids the trouble of getting the solution of nonlinear equations, and it can satisfy the precise demand of ADC. The analysis of testing example shows the efficiency of this method.