The tensile properties of tri-component Sirofil composite yarn is analyzed. To characterize the stretchability of the composite yarn, a three-element model with little distortion has been established. It is concluded that the elasticity modulus can be improved when three single yarns with different modulus of elasticity are composited. Under this condition, the modulus of elasticity of the composite yarn is among the three. Furthermore, in order to describe the elongation of the composite yarn at break, a four-component dynamic model of drawing with large distortion is developed. And the results obtained are verified by non-linear regression analysis. It is demonstrated that the properties of non-linear visco-elasticity of sirofil composite yarn can be characterized by using multinomial and the curve plotted according to the experimental result is basically in agreement with that deduced by the theoretical model.