Abstract: In order to achieve high-precision measurement of refractive index of optical materials, the minimum deviation angle method was improved using three-image method. By separately measuring three minimum deviation angles corresponding to apex angle of prism, precise positioning of the minimum deviation angle was achieved, thereby improving the measurement uncertainty of refractive index. Comparative experiments on refractive index measurement using minimum deviation angle and three-image method were conducted on angle measuring instrument to verify that three-image method can enhance measurement uncertainty of refractive index, with a theoretical uncertainty reaching 10⁻⁶. Considering the influence of actual environmental conditions on test results, an improved environmental correction formula was derived.