A detailed theoretical derivation of the principle of measuring the refractive index of glass by the V prism method is carried out. The precision of measuring the refractive index of glass example by the V prism method can reach ±0.000 05 . At the same time, according to a large number of glass composition and glass refractive index test data, the fast calculation formula of the refractive index of CaO-B2O3-SiO2-Al2O3 E-glass was fitted out: n′=1.685 57ωCaO+1.385 31ωB2O3+1.536 71ωSiO2+1.612 23ωAl2O3, the precision is ±0.005. The method is used to study the influence of glass composition on the refractive index.