In the present paper we introduce the self-collimation method to measure the geometric parameters of solid immersion lens (SIL). A laser beam collimated by an objective is impinging upon the SIL. The reflection beams from the spherical surface and bottom plane can be Changed into self-collimation beams after passing through the objective when the SIL is put at three parti cular axial positions. The spherical radius,index and other parameters of the SIL can be obtained from these positions. The expression of self-collimation positions and experimental results of two SIL are given.