The traditional cutting and extrusion methods for processing thin-walled cylinder with longitudinal external ribs have the problems of low material utilization and high cost respectively. So a new composite flow forming process technique was proposed. By using finite element analysis software, the influence laws of process parameters such as pre-forming reduction, pass thickness reduction distribution ratio and ball diameter on forming effect of external ribs were studied. The result shows that the bigger the pre-forming reduction is, the better the filling effect of external ribs is. The influences of ball diameter on filling effect of ribs are different in different stages. When using two passes, the axial spinning pressure during the process can be significantly cut down. The filling rate of external ribs and spinning pressure were taken as indexes. The best process scheme of composite flow forming for thin-walled cylinder parts with longitudinal external ribs was obtained by orthogonal test. The optimal process scheme was selected and the feasibility of the composite flow forming process was verified by experiments, which provided a theoretical design basis for the efficient and low-cost production of such components.