The structure of three sub-mirrors sparse aperture two-mirror system is introduced. The generalized pupil function and imaging principle of the system are derived. One of the three sub-mirrors has piston error and tilt error. The errors are represented by Zernike polynomials, which are imported in simulation of imaging with Matlab. Relative ambiguity resolution is advanced to evaluate imaging quality of the system. Through the relationship between the errors and the relative ambiguity resolution, the influence of sub-mirror errors to imaging quality is analyzed. The results show that the impact on imaging quality with piston error is different from that with tilt error. With the increase of the error, the relative ambiguity resolution periodically changes with piston error but monotonically increases with tilt error. Tilt error affects the quality of imaging more than piston error does, so we should control tilt error more strictly than piston error.