Abstract: In This article a space model of the relative link length types of the four-bar linkage is first derived, and divided into eight subregions contrasted with eight families of mechanisms. Hence we can subdivide the whole model into 48 small regions and get 48 sorts of four-bar mechanisms. Taking any section of the space model, there is a closed region in plane, and we can plot several character diagrams of the mechanisms in it. These character diagrams are useful to realize the relation between the mechanism characters and the link length in the total, and consequently are helpful to improve the synthesis method of the mechanisms. At last the practical meaning of the space model is shortly discussed.