This paper proposes a form-finding method of free-form aluminum alloy reticulated structures with semi-rigid gusset joints. The vertical coordinates of the control points of the B-spline surface are selected to be the design variables adjusting the shape of the structure. The joint gap index, which is proposed for characterizing the manufacturing and assembling convenience of aluminum alloy structures, the strain energy, and the safety factor under short-term snow load, are chosen as the objective functions of the multi-objective optimization problem. The Pareto optimal solutions of the numerical example indicate that slight differences in the structural shape can result in quite different mechanical behavior, and the NSGA-II algorithm is effective in providing a wide range of optimal structures as compromise solutions of the multi-objective problem. Finally, the same optimization problem is solved for rigid jointed structures to study the influence of the joint rigidity on the formfinding result. By comparison of the internal force distribution, it is found that the joint semi-rigidity will influence the optimal structural shape due to the difference in the load-bearing mechanism. Therefore, it is suggested that the actual joint rigidity should be considered in the structural optimization process of reticulated structures.