The structural composition of long-span cable-stayed bridges in mountainous area is complex. As the static behavior of the whole structure under live loads presents obvious nonlinearities, the structural stability problem is prominent. Therefore,based on a steel truss girder cable-stayed bridge with a main span of 930 m under construction in the south mountainous area of Yunnan Province, China, the ultimate load carrying capacity analysis is carried out by using limit point instability theory where both structural geometric and material nonlinearities are considered. It is aimed at investigating the nonlinear behavior and failure mechanism of such a long-span bridge under live loading. Because of the existence of “canyon effect” for the wind in the mountainous area, the bridge is easy to be affected by wind loads during construction. Subsequently, the ultimate load carrying capacity analysis of the bridge at the maximum cantilever construction state is performed under static wind loading. The results have shown that the geometric nonlinearity has less influence on the structural performance than the material nonlinearity does. The ultimate load carrying capacity of the long-span cable-stayed bridge is controlled by the material failure of the cable. With the increase in live load, the elastic-plastic zones develop gradually. The tensile and compressive plastic zones appear successively on the steel truss girder, forming four yield paths. At the maximum cantilever construction stage, the calculated ultimate load capacity is greater than static wind load of Grade 12 wind speed.