Abstract: In this paper, the relationship between the emergent light ray angle and incidence point is investigated according to the principle of the geometric optics and the computer simulation experiment so as to obtain the dependence of the incidence point on the vertex angle of the prism, refraction index and the basal-plane length of the prism from MATLAB. The experimental result shows that a certain incident point of the incident light is required for the measurement of reflection index of a prism and the application of a spectroscopic instrument.