The Gauss width of focal spot which is too small results in the sampling error while using CCD as the centroid detecting device of Shack-Hartmann Wavefront Sensor (SHWS) in Adaptive Optics (AO) system. In order to solve the problem of sampling error, the reason of sampling error of CCD is analyzed in detail. The math model of sampling error, displacement sensitivity and open loop dynamic range are established. The results show that the optimal position during sensor calibration is the center of the four pixels of sub-aperture, where centroid detection possesses that sampling error is 0 and the displacement sensitivity and the open loop dynamic range are maximum as well. It provides theoretical basis for research of SHWS calibration method in AO system.