The Segmented Telescope is the most effective way to build a giant telescope. 10 meter and larger telescopes are needed to be carried out further for our country. The analytical expression of the Point Spread Function (PSF) for 10 meter Segmented telescope was derived, based on optical diffraction theory. A numerical simulation model was established. The Gaps of Sub-segmented mirrors and Piston error, which affect far-field image quality, were analyzed. Both theoretical analysis and simulation results demonstrated the following: PSF of Segmented telescope can be obtained by multiply coherence function(GF) of Sub-segmented mirrors′ center position with single Sub-segmented mirror′s PSF. Gaps between Sub-segmented mirrors could blur diffraction spot and decrease the peak energy. Piston error of the Sub-segmented mirror could affect the GF of the PSF seriously. Thereby, Strehl Ratio of the system could be decreased and the frequency of GF is not changed. Tip-Tilt error of Sub-segmented mirrors would cause frequency shift in every PSFs and the whole PSF could be affected and the Strehl Ratio would be decreased.