This paper focuses on the prediction of the formability limit of the stiffened panels during the loading stage and heating stage of the creep age forming (CAF) process. A set of non-linear finite element (FE) simulations has been carried out to investigate the elastic-plastic buckling behaviour of stiffened panels subjected to pure bending at the room temperature and the ageing temperature (155 ℃) for AA2050. From the non-linear FE simulations, the effects of main geometric parameters (stiffener thickness and stiffener height) on the critical buckling stress and the critical radius of the stiffened panel have been investigated at both temperatures. Increasing the stiffener thickness or decreasing the stiffener height leads to an increase of the critical buckling stress and an increase of formability limit. The critical buckling stress at the ageing temperature is smaller than that at the room temperature when the stiffened panel enters the plastic region, however, the formability increases slightly at the ageing temperature.