A method for obtaining the value of light ring of the plane optical element is presented. Three tested zones and searching zones in the interferogram are determined to search the average curvature of the interference fringes and the average fringe spacing, and the optimum estimation value of light ring of the plane optical element is obtained by using the weight coefficients of the tested zones. By measuring the standard optical elements and comparing with the visual interpretation results of interference fringes, the maximum relative error of 5% is obtained. Experimental results show that the method can be utilized to improve the adaptation capability of processing interferograms and substitute the visual interpretation of interference fringes.