Based on the vectorization of grating equation, two-dimensional scope of grating diffraction is extended to three-dimensional space. The predictability of direction of incident light diffracted by optical disc in three-dimensional Euclidean space is quantitatively given. The grating vector is introduced to quantitatively explain the necessity of color line-line structure on the surface of optical disc under the condition of long distance. The spatial light cone is introduced to quantitatively calculate the principal maximum supremum of diffraction, which has a good visualization effect. Combining the dispersion power, overlap phenomenon, deep well effect and parallax angle, in this paper the problem of how to diffract more pure color lines by optical discs is solved, the commonness and difference between CD and DVD is given, and the theory is found to consistent with the experiment result.