In order to disscuss the clinical signification on expanding skin flap with papaverine cream and to offer a theory for its application and popularization ,the study was to evaluate the constitutive relation and to find an expression for its biomechanical properties by mathematical methods. Small porines were were divided into two groups: medication expansion group, conventional expansion group. Papaverine cream was applied externally in medication expansion group during skin flap expansion. The stress-strain was measured in each group during skin flap expansion and was fitted with theoretical curves and constitutive equations, and the mathematic model was obtained. An expression for the constitutive relation of expanding skin flap was found by the exponential function type of constitutive equation and that of theoretical curve. The viscoelasticity of medication expansion skin flap trended to enhancement as compared with that of conventional expansion skin flap. The results indicates that medication expansion skin flap can reduce the increase on tissue tensile force in some degree and benefit wound concrescence and expand repair area when expanding skin flap transplanted.