Abstract: From the thin lens imaging formula, the formulas of the front focal length and the back focal length of the thin lens are deduced, and the changing law is examined of the focal length when the medium refractive index changes by a case study of the positive lens and negative lens. Results show that when the refractive index of the medium is less than that of the lens, concave lens play a convergent role, and the convex lens play a divergent role. But when the refractive index of the medium is greater than that of the lens, concave lens play a divergent role, and convex lens have a convergent effect. The experimental investigation shows that the experimental result is consistent with the theoretical conclusion.