Constant deviation prism is a turning element which can invariably transform the propagating direction of the light beam.One kind of constant deviation prism is analyzed in this paper.Its ray tranfer matrix and misaligned matrix are given,under the condition of manufacture tolerance,its transfer matrices are also given.And it is shown that it is a two dimensional insensitivity element.At last,the propagating characteristics of the general off axis astigmatic Gaussian beams through this element are analyzed,the tensor transfer laws of the complex curvature and the off axis parameter are given.