Basic structure of GiresTournois mirror is described and the dispersion performance is calculated. The factors affecting the performance of the GiresTournois mirrors are discussed. The results show that the layer number of high reflector affects the reflectance of the GiresTournois mirrors but the thickness of the GiresTournois cavity and the layer number of the top reflector affect the dispersion performance of the GiresTournois mirrors; to achieve good design performance, the layer number of high reflector, the thickness of the GiresTournois cavity and the layer number of the top reflector are selected to be 40~60, λ/2 or λ and less than 5.