The principle and design of a cat′s eye made of two glass hemispheres are described. It shows that cat′s eye becomes a whole sphere when the glass with refractive index of 2 is used. The optical path analysis, indicates that the offset between two hemispheres (including the thickness error of the glue) play an important role in the final accuracy of the laser tracking interferometer. The relation between divergence angle of the reflected rays, radius of the hemispheres, refractive index and incident height of the entering rays is discussed. The parameters of cat′s eye are optimized to get the least divergence angle of the reflected rays, and the optimal parameters are given.