A pretension design method is proposed for the cablenet system of large span cable-beam structure. This method can improve computational efficiency of pretension design under influence of beams’ deformations. In this method, cablenet system is divided into two parts-inner cablenet and edge cablenet. Inner cablenet is a pure cablenet area, its optimal pretension distribution can be easily found by using balance matrix analysis method. Edge cablenet is the other cablenet area connected with beams, and for this cablenet system, an iterative calculation combined with balance matrix analysis method and nonlinear finite element method is used to calculate its reasonable pretension distributions. In the iteration process, the shape of edge cablenet is iteratively updated according to displacements of connection joints between edge cablenet and beams, and based on these shape changes, pretension distributions of edge cablenet is recalculated iteratively. And by using this method, influence of beams’ deformations can be added to cablenet pretension design. Because above complicated iterative calculation is just limited to the edge cablenet, scale of nonlinear finite element calculation in the iteration will be much smaller than calculation for the whole cable-beam structure, and the efficiency and accuracy of this calculation can be also improved at the same time. At last, this method is programmed for a numerical example, and the results indicate that the method is feasible.