Effect of the size and shape of cup shaped forgings with flange on forming capability of aluminum alloy was investigated. The numerical simulation was carried out on the extrusion forming process for each part using two-dimensional elastoplastic finite element method. The metal flow behavior was analyzed, and the forming defects were predicted. Meanwhile, the corresponding experimental researches were conducted. The results show that the forming defects like folding or crack appear in each aluminum alloy forging. The size and shape of cup shaped parts have a great influence on the type and size of forming defects. Good agreement is found between the numerical results and experimental work. The results present important references for technology analysis and mould design.