Abstract: An improved design method is proposed to overcome the disadvantages that the current design method lacks sufficient considerations of the structural system reliability under asymmetric loads. First, the characteristics of a load effect function are discussed under asymmetric loads. Second, a general model of the load effect function is obtained through the analysis of some structures. Based on the function model, the limit state equation can be expressed explicitly at the structural system level. Furthermore, the nonlinear characteristics of a limit state surface are analyzed in a standard normal space. Finally a group of optimal values of partial factors is calculated according to the target reliability index. It provides some references on the system reliability design under asymmetric loads for long-span spatial structures.