To realize the lightweight design of the combinatorial jib, the design theory of the combinatorial jib was analyzed, and the sectional dimensions and the limb spacing were taken as design variables. Based on the rigidity, strength, and stability of the combinatorial jib, the mathematical model of lightweight design of the combinatorial jib was established. Then, a practical problem was studied. Moreover, the mathematical model was solved, and its lightweight design solution was obtained. The results demonstrated the validity of the solution, and it provided a certain way to realize the lightweight design of the combinatorial jib in engineering.