In this paper, we suggest a grazing incidence mirror system, which consists of a pair of co-axial, non-confocal paraboloid and hyperboloid, instead of the current co-axial, confocus system (i.e. Wolter-I type). By introducing a given amount of spherical aberration into Wolter-I type by means of non-confocus, a part of off-axial spherical aberrations can be compensated. Compared with Wolter-I type, the image quality of this non-confocal system has been improved in the angle field of 0~15 arc min. Results of computation show that MTF is more suitable than line spread function (LSF) or root of mean-square of blur circle (rms) as the criterion for evaluating the image quality of such systems. The set-up of flare stops are considered. Tolerances on the geometrical dimensions of the mirror are analyzed. Heretofore, the error of the size of apertures is represented as a relative shift between the focus and the edge surfaoe of the mirror, and then the image quality is computed accordingly. Substantially, the results are in agreement with the published data. The tolerance of difference of mirror apertures and the tolerance of circularity are more stringent than the tolerance of the size of apertures at least by one order of magnitude. The tolerance of difference of mirror apertures for paraboloid is smaller than that for hyperboloid by a factor of four or five.