Based on optical diffraction theory and geometry optical, such as diameter of diffraction spot, Point Spread Function (PSF), Strehl Ratio, Modulation Transfer Function (MTF), etc., were analyzed using theoretical analytical expression and numerical simulation of segmented telescope in the paper. The effect of sub-segmented mirrors’ Piston error and Tip-tilt error on far-field was calculated. The results show that both Piston error and tip-tilt error affect system’s Strehl Ratio and MTF and blurred far field image. Tip-title error can cause the generation of piston error. The compensation for the Piston error would highly improve the far field imaging characteristics.