1. A three-dimensional model retrieval apparatus comprising: at least one processor communicatively coupled with said apparatus including: a model normalizing unit configured to rotate and/or translate a three-dimensional model so that main axis directions of the three-dimensional model are consistent with coordinate axes of a three-dimensional space, and barycenter of the three-dimensional model is consistent with origin of a coordinate system; a two-dimensional image generating unit configured to project said three-dimensional model respectively in a positive direction and a negative direction of each coordinate axis of said coordinate system to generate a plurality of two-dimensional images; a model describing unit configured to generate a model descriptor of the three-dimensional model from said two-dimensional images; and a retrieving unit configured to retrieve, based on said model descriptor, a three-dimensional model which most matches an input query from a model database, and where said model normalizing unit calculates a normal direction of each triangle constituting said three-dimensional model, determines the main axis directions of the three-dimensional model based on distributional information of the normal directions of all triangles, and rotates the three-dimensional model based on the main axis directions, and a weighted distribution matrix of the normal direction is calculated, the matrix is subjected to Eigen-decomposition to obtain three eigenvectors, and the three eigenvector are taken as the main axis directions, and wherein the matrix is represented M = ∑ fCnt ⁢ ⁢ a i * ( p i → - p 0 → ) * ( p i → - p 0 → ) T where {right arrow over (p 0 )} indicates an average value of the normal directions, a i and {right arrow over (p i )} respectively indicating the area and normal direction of each triangle, fCnt indicates a number of triangles contained in the three-dimensional model, and T indicates a transposing operation of the vector.