A novel method to measure the refractive index distribution of two transparent mediums was proposed in this paper after analyzing measure methods of refractive indexes in existence. The basic principle and interferometry method of the measurement of micro refractive index change between two transparent mediums were described, and the theoretical feasibility of this measurement method was studied by using Zemax optical simulation software. Firstly, in this experiment, a composite plate from two transparent mediums to be measured was prepared by adhere-free bonding technology. Secondly, the optical path difference caused by the refractive index difference in the bonding interface was measured with the help of interferometer, under the different splitting angles of the composite plate. And then, the relational expression between the refractive index distribution and the optical path difference was deduced by using the law of refraction in geometrical optics. At last, the micro difference value of the optical path difference for normal incidence was obtained by conic approach technique in dealing with measuring data, and thus the final result of refractive index difference between two transparent mediums was obtained according to the conversion expression. The measured value of refractive index difference of a 1 at. % Yb3+: YAG/YAG bonded sample was also gotten and the value was 5.41×10-5, thus the accuracy of measurement system was up to 10-7 magnitude. The experiment result shows that the measurement method is simple, easy operation and has high precision.