Abstract: On the basis of the FDUC model and the hypothesis of the constant covalent radii,the expressions of the atomic near- neighbor and next near neighbor bond lengths are derived for the quaternary Ⅲ-Ⅴ quaternary solid solutions.Besides depending on the concentration(x,y)of the four elements,this set of bond-length's expression is only dependent on the lattice parameters of the four pure end-compounds(or the covalent radii of the four Ⅲ-Ⅴ elements).And it also covers the case of the relative pseudobinary solid solutions.Further discussion and comparison with other theoretical models are also given in this paper.