The two-point resolution of optical image system has long been introduced in several literatures. The process was not only based on diffraction effects, but also on the sine condition in spherical centred system, which is not held in ideal optical system according to Gaussian optics. In the paper, the view point is used that image point determined by Gaussian optics, is formed by diffraction effects resulting from the exit pupil according to Rayleigh theory. The viewpoint could be regarded as basing on linear system, as diffraction-limited imaging system is essentially in accord with linear system theory. The two-point resolution formulas of linear optical image system are derived under the viewpoint, and one of the conclusions is that even if the diameter of the exit pupil, or the entrance pupil, is infinite, 0.61λ is the distance between two object points which can be just resolve.