Abstract: The nonlinear characteristics of the distribution and propagation of initial microcracks in reinforced concrete structures are obvious. It is of great significance to reasonably estimate the damage range and crack width in the process of concrete deterioration to reduce the economic loss of the project and ensure the safe operation of the structure. On the basis of establishing four-parameter equivalent strain and four-parameter failure criterion of strain space, the damage variable of concrete structure under complex multiaxial stress state is solved. After the relevant information of discontinuous cracking part of concrete is obtained through the strain decomposition method, the expression of crack width in the whole process of structural damage and cracking is deduced according to damage mechanics and smeared crack theory. Taking a four-point bending beam as an example, the finite element calculation is carried out. The research results show that the above method can describe the deformation characteristics and cracking rules of the reinforced concrete beam during the whole stress process from loading to failure in detail. The numerical simulation results are in good agreement with the test data, which verifies the rationality and accuracy of the calculation model. The research results can provide decision-making support for the maintenance and reinforcement of reinforced concrete structures in practical projects.