In order to simulate stress-strain curve of duplex stainless steel, the austenite and ferrite stainless steels with single phase, whose compositions are similar to those of austenite and ferrite in SAF 2205 duplex stainless steel, were designed. The stress-strain curve of the duplex stainless steel was calculated with ANSYS finite element (FE) software, and compared with the measured one. The effects of the volume fraction of austenite and the ratio of austenite and ferrite yield stress on mechanical properties of the duplex stainless steel were discussed. The calculated stress-strain curve of duplex stainless steel based on that of single phase austenite and ferrite stainless steel is coincident, with the measured one very well. The volume fraction of austenite affects plastic deformation behavior of the duplex stainless steel. With increasing of volume fraction of austenite, the strain can be increased greatly for same applied stress. The yield stress ratio (C*) between austenite and ferrite in duplex strainless steel also affects its plastic deformation behavior. With the increasing of C*, the strain can be increased slightly for same applied stress.