According to the principle of total internal reflection and autocollimator angle measurement method, accurate measurement of the prism refractive index is achieved. By the using of highly aligned semiconductor laser, the laser goes into interface of the prism and air. The curve of the prism′s reflected light versus the incident angle is obtained by rotating prism or changing the prism′s incident angle gradually. A step appears at the left side of the curve ending and its reflected light rapidly attenuates with the increasing of the incident angle, which is the angle of total reflection. The base angle of the prism is accurately measured by the double using of auto collimation angle measurement method. Refractive index of two pieces of different prism is measured by this method, respectively. The measuring accuracy is ±1.24×10-4.