Abstract: This article discusses the pull-out mechanical model of NiTi shape memory alloy (SMA) fiber. In constant temperature state , considering the interfacial f riction between the fiber and mat rix , the tensile problem of SMAfiber under varying axial force is discussed , and the formulation of memory alloy fiberps pull-out length is obtainedby two theories respectively , namely the index model of Tanaka and the cosine model of C. Liang. The numericalresults are given. The good agreement between the results of index model and cosine model warrants this new theoretical method.