Pseudoelasticity and shape memory effect(SME) are the most fundamental properties of shape memory alloys (SMAs), which can be attributed to the combination of the individual property of each phase of SMAs that are dynamically composed of austenite and martensite. Based on the theory of mixture, a constitutive model is proposed, in which, the behavior of austenite is further assumed to be linear elastic while that of martensite elastoplastic. The behavior of SMA under different temperature is described and compared with experimental observation. Compared with the existing model, the proposed model is simple, physically distinct and can easily be applied in practical engineering problems.