The vast majority of cases in optical surface testing，the output surface of measured optical surface or optical system always tends to be smooth and continuous, such wavefront functions must can be expressed as a complete the linear combination of the basement function. Zernike polynomials are often used as basic set of polynomials for the fitting of measured discrete data. The wavefront and curved surface are interpreted in the form of linear combinations of Zernike polynomial. The effects of fitting precision owing to Zernike polynomials order and sampling points are studied. The result is obtained that higher order Zernike polynomial fitting wavefront is not better，if order more than a certain range，fitting results will appear sick. Therefore, the wavefront fitting should choose appropriate order of Zernike polynomial. When the polynomial order is selected, more sampling points will help improve fitting accuracy. However，the number of sampling points is not a prerequisite for improving the fitting precision.