An approximation method for directly solving the relative light intensity of centers of bright stripes in Fraunhofer single slit diffraction is presented，and an indepth error analysis is conducted. By combining halfwave zone method and amplitude vector method，the central light intensity of bright stripes can be calculated simply and directly. By comparing different amplitude vector graphics，the error cause is analyzed. The accurate positions of bright center are obtained through quantitative analysis. It is found that the maximum relative error of this method is less than 5 %，and when the fringe order is greater than or equal to 3，the relative error is less than 1%.