For testing the concave ellipsoidal mirror of Three Mirror Cassegrain (TMC) optical system, a slab compensated auto-collimation method is suggested. This method can avoid the problem that the performance of null compensator is hard to be evaluated, and the development of null compensator cost too much. Taking advantage of the shape of TMC system’s primary mirror near to parabolic, slab compensated auto-collimation method is analyzed to test it. Expression of spherical aberration in testing optical system is deduced by conic aspheric normal character, and the parameters of compensated slab are given by solving minimal residual spherical aberration. For test of some actual Φ500 mm mirror which has 1 589 mm vertex radius and -0.983 conic coefficient, a slab compensator with only size Φ34.2 mm× 9.126 5 mm is designed. The wavefront aberration of test optics compensated by slab is only 0.003λ, matching the demand of precise test. The test, manufacture and use of the slab are also considered. The slab compensated auto-collimation method is obviously low-cost, and it is easy to evaluate the performance of the slab.