High accurate determination of refractive index of optical glass has become more important aspect to ensure imaging quality of optical systems. In this paper

we introduce the measuring method of refractive index of optical glass

both foreign and domestic actuality of development .We bring forward a new method to calculate refractive index by measuring the three least deflection angle of three vertex angle of prism

the characteristic of method is that accuracy of the refractive index measurement improve with refractive index augment. Accuracy of the refractive index measurement is more than 1x10-6 by goniometer which accuracy of angle measurement more than 0.5². Formula of new method is deduced ， the accuracy of measurement are analyzed in the theory. New method have better application foreground because of its high accuracy.