In the assembly process of the encoder circular grating，the installation distance between the rotary central of the spindle and the base circle center of the code of circular grating has a great influence on the output precision of the encoder signal. In order to improve the detection efficiency and detection accuracy of the circular grating eccentricity detection process，a new method for calculating the angle position and the value of the eccentricity of the encoder is proposed. Through the feature extraction and data analysis of the coded image of grating，the maximum of abscissa in different position of code basic circle is calculated，the angle position and the value of the eccentricity are obtained. Experimental results show that this method can effectively calculate the eccentric position and angle of the grating. The adjustment system of this method has a higher precision and the adjustment efficiency，and can reduce the influence of the processing error on adjustment results.