The plastic anisotropy R-value is an important material parameter for constitutive models, and the fact that the R-value varies with deformation has been recognized. However, there is not a unified method so far to determine the variable R-value. Therefore, based on the initial definition of the R-value, this paper proposes a reverse fitting model to determine the evolution of the R-value for sheet metal. Meanwhile, the limitations of the existing calculation methods for the R-value have been compared and analyzed in detail. The results show that although the longitudinal plastic strain-transverse plastic strain curve is closely to a straight line, the R-value changes greatly. With the increase of the uniaxial tensile plastic strain, the evolution of the R-value is generally decrease, and the curves form can be divided into two types: up convex and down concave. In addition, the reason why there are significant differences of the R-value calculated by different methods is clarified. The proposed reverse fitting model can greatly simplify the secondary development of constitutive model considering the evolution of anisotropy, and enhance the accuracy of numerical simulation.