The project Expo Axis is a landmark building of Expo Shanghai 2010. Its roof system is composed of a cable-membrane structure and six free-form single-layer latticed shells named Sun Valley. Sun Valleys are different in shape and size and provide eighteen support points for cable-membrane structure. Because there is no clause used for free-form structure in current codes and specifications, it is difficult to carry on stability analysis for Sun Valleys. The Sun Valley NO.1 is studied for overall stability analysis in the paper. Ninety three load combinations are used in the process of buckling analysis, and D+0.7×L2+W315 is selected as control load for nonlinear overall stability analysis. And then with different initial conditions considered the influence of geometric nonlinearity, plastic properties of material and initial geometric imperfection on overall stability of the structure are researched. The results show that the stability load factor of structure can meet the relevant specifications of China in every condition. Some members enter the plastic state when first critical load is reached, so the structure will enter strength failure before overall unstable failure happen. The outputs of the study will provide useful reference for design and analysis of other free-form projects.