Abstract: This paper focuses on the arch rib system of a leaning-type arch bridge which is a new type space arch bridge developed in recent years, the following investigation is carried out: Based on the principle of minimum potential, as well as the deformation energy of main ribs, stable ribs, the transverse brace between the main rib and stable rib, and the potential energy of an external force applied on the ribs system, the lateral buckling critical load of a leaning-type arch rib system is derived in this paper, by which the calculation method for the lateral buckling critical load is proposed. Compared with the results obtained from FEM results, it is verified that the method presented in this paper is accurate sufficiently. Furthermore, it is analyzed how those design parameters such as variation of incline angles, the lateral flexural rigidity and torsional rigidity of a stable rib and a transverse brace between the main rib, the stable rib influence on the lateral buckling critical load. The investigation shows that the incline angle and lateral flexural rigidity of stable rib are main factors to ensure the lateral buckling of a leaning-type arch rib system.