Abstract: The matrix damage evolution behavior and the effect of matrix micro-cracks on the dynamic characteristics of composite stiffened delaminated plates are studied. A stiffness reduction model considering matrix micro-cracks criteria is established. The model can be used to investigate the matrix damage evolution behavior under harmonic excitation effectively as the influence of the number of cycles has been taken into account. On the basis of Mindlin deformation assumption and the modal damping model, a dynamic numerical analysis for investigating the characteristics of damage evolution, stiffness reduction, natural frequency reduction and dynamic response of the delaminated stiffened composite plates has been carried out by the finite element method.