Abstract: On the basis of Hsieh-Ting-Chen failure criterion, a four-parameter equivalent strain for isotropic damage modulus is presented. The four parameters are determined in terms of the material characteristic of the concrete sample and the available experimental strength data. The equivalent strain transformed from strains of different stress states is calculated. The corresponding damage modulus is then obtained. A cubic concrete sample is analyzed with finite element method. Results show that the computed stress-strain curves agree with the experimental stress-strain curves and the rationality of the equivalent strain is verified. It is also showed that the damage model with the four-parameter equivalent strain can describe the compelte failure curve under uniaxial stress state and multi-axial stresses state. The equivalent strain is simple and easily obtainable.