In the current work, an inverse analysis on the primary shear zone was introduced to determine the five constants in Johnson–Cook’s material constitutive equation under the conditions of metal cutting. Based on the detailed analysis on the boundary conditions of the velocity and shear strain rate fields, Oxley’s “equidistant parallel-sided” shear zone model was revisited. A more realistic nonlinear shear strain rate distribution has been proposed under the frame of nonequidistant primary shear zone configuration, so that all the boundary conditions can be satisfied. Based on the presented analysis, the shear strain, shear strain rate and temperature at the main shear plane were calculated. In conjugation with the measured cutting forces and chip thickness, a genetic algorithm (GA) based optimization program has been developed for the system identification. In order to verify the effectiveness of the developed algorithm, the obtained material constants were used in a forward analytical simulation. The acceptable agreement with experimental data validates the proposed method.