Abstract: The method of improving the accuracy of the diagnostics with alignment pointer is introduced. The transmission characteristics of the laser coupled by the single-mode fiber and passing through the optical system of the alignment pointer were analyzed based on the transformation principle of Gaussian beam thin lens. The paraxial magnification is the major factor to affect the spot size in image space of the alignment pointer. The optical system of the alignment pointer for SG-Ⅲ diagnostic package was designed, whose illumination source was a fiber laser, working wavelength was 635nm, total length was less than 100mm, target distance varied from 600mm to 1500mm, and spot was located at the center of the target and its size was 46.2μm～71.9μm. The optical system includes three normal glasses, in which the fixed group is composed of two separate positive-negative glasses, and the zoom group is a negative lens. The quality of the optical system was evaluated by the point spread function and wavefront aberration. The results show that the optical system meets the design specification.