The reason of "correlativity" or "ill-condition" in the fitting calculation has been found and a basic rule to fit interferogram with Zernike polynomials has been obtained by systematically analyzing the fitting method and course, i.e. the order of Zernike polynomial to fit interferogram should be less than the number of interference fringes in the diaphragm. By the rule " correlativity" or "ill-condition" in fitting calculation can be absolutely avoided, thus the reliability of interferometry is ensured.