A method on how to decide the resolution of Shack-Hartmann sensor when it is used to measure an unknown incident wavefront is presented. The phase of incident wavefront can be according to Fourier progression, the complex amplitude of incident wavefront can be expanded according to Bessel progression, and then the distribution of intensity of far-field spot can be got through Fourier transform. The results indicate that the phase spectrum of incident wavefront have affiliation with the distribution of intensity of far-field spot. The resolution of Shack-Hartmann sensor can be decided through measuring the distribution of intensity of far-field spot. This method provides the theoretic support on the decision of the number of sub-aperture of Shack-Hartmann sensor. So the problem of the decrease in accuracy of Shack-Hartman sensor because of the lack of sub-apertures can be solved.