A six-parameter mathematical model was introduced to simulate the stress-stain hysteresis and the inner hysteresis of polycrystalline shape memory alloys (SMAs). By comparing with experiments of Cu-Zn-Al SMA, it was shown that the model could be used to calculate the stress-strain relations with rather good accuracy. Moreover, it was found that the six parameters introduced in this paper represented the characteristics of the stress-strain hysteresis of polycrystalline SMA and can be used to characterize the hysteresis quantitatively. A six-parameter mathematical model was introduced to simulate the stress-stain hysteresis and the inner hysteresis of polycrystalline shape memory alloys (SMAs). By comparison with experiments of Cu-Zn-Al SMA, it was shown that the model could be used to calculate the stress-strain relations with rather good accuracy. Moreover, it was found that the six parameters introduced in this paper represented the characteristics of the stress-strain hysteresis of polycrystalline SMA and can be used to characterize the hysteresis quantitatively.