In an interferogram data processing, the characters of base function forming wave aberration polinomials, the solving process of canonical equation, the number of sample points and their distribution all have their influence on the solution of wave aberration functions, as well as the digitization error introduced inevitably into the measurement of interferograms. These errors are quantitatively analysel based on statistics and represented in the way of computer simulation. Meanwhile, the quantitative criteria about the selecting base function and number of terms of polinomials, the number of sample points and their destribution, and the rationality of fit results, are prosented.