In order to improve the computational efficiency of sub-pixel location of a diffraction spot generated by a two-dimensional phase plate with the correlation function fitting extreme method, three effect factors,the template size, fitting window size and the correlation function were analyzed in detail. On the basis of the theory of emulation image examination, mathematical emulation images were used to verify the reliability and accuracy of algorithms and programs. Analyzing results indicate that the three effect factors all have bigger effect on computational efficiency and the template and fitting window sizes show the biggest effect on the locating precision during integer pixel searching. The optimal design of searching template and fitting window sizes are completed during the sub-pixel location experiment of diffraction spot generated by the two-dimensional phase plate,It is shown that the optimized template and fitting window sizes are 51 pixel×51 pixel and 3 pixel×3 pixel, respectively. Comparing with the template of 131 pixel×131 pixel which just includes the whole diffraction spot, the calculating time of whole locating process is shortened by 40%, which shows the computational efficiency is improved without degrading location precision.