The impact of segmented telescope piston error, tip-tilt error and gap on image quality by theoretical and numerical simulation was analyzed. Different error segmented telescope Point-spread-function (PSF) and the far-field spot Strehl Ratio (S.R) expression were derived theoretically. Then, set up simulation model about three kinds of error effect on segmented telescope PSF and far-field spot S.R, and analyzed three kinds of error influence on imaging quality of splicing telescope. Analysis results show that: when the segmented mirror exist piston error, segmented telescope image quality with piston error standard deviation δ increasing gradually becomes poor; when δ < λ/20 segmented telescope, the far field spot RS.R >0.9;when the segmented mirror exist tip-tilt error, the segmented telescope imaging quality with the wavefront phase RMS increases gradually worse, when eRMS<0.3 rad, the far-field spot of the segmented telescope RS.R>0.9; when the segmented mirror exist gap, its imaging quality with the gap factor w increases gradually becomes poor, and when w<0.025, a segmented telescope far-field spot RS.R>0.9.