In order to measure accurately the refractive index of optical glass to improve the imaging quality of optical systems,several measuring methods of refractive index for optical glass were analyzed and both foreign and domestic development situations for optical glass measurement were introduced.A new method to calculate refractive index by measuring the three least deflection angles of three vertex angles of a prism was presented.Proposed new method made full use of the orientation dominance of complementary method or three-image method improve the accuracy of refractive index measurement,and it was characteriszed by the accuracy improvement of refractive index measurement with refractive index increasing.The formula of new method was deduced,the measurement accuracy was analyzed in the theory.Result indicates that measurement accuracy has improved more than twice as compare with that of traditional least deflection angle method,which can meet the requirement of high accuracy measurement of refractive index.