The preload and precompact capacity of a two-piece outer ring self-aligning ball bearing are analyzed in this paper to ensure the preload effectivity. This bearing has the merits both from a double row angular contact ball bearing and a double row self-aligning ball bearing, so it not only can be preloaded to enhance stiffness, but also can self-align to compensate the misalignment. The maximum misalignment angle reaches 3°. In order to be preloaded properly, the outer ring is consisted of two spherical inner face raceways, which forms a spherical surface whose center is common to the bearing center and a in-between spacer. The preload is controlled by adjusting the thickness of the spacer. The relationship between preload and bearing supporting is analysed and formulated with the help of a mechanic theory. Meanwhile, based on the Hertz contact theory, the formula to determine the spacer thickness is given. This kind of bearing is adopted in a LAMOST telescope,test results show that the radial runout is less than 0.02 mm and axial runout is less than 0.03 mm for a φ1 100 mm bearing at an arbitrary angle of 50°. The tracking accuracy of focal rotation is better than 0.3″supported by this kind of bearings.