Abstract: The transient response of delaminated composite laminate under low-velocity is discussed in the present paper. At first, the Mindlin plate finite element is used to describe the movement and deformation of composite laminates with a circular delamination, in which large deformation is considered. Secondly, an improved Hertzian indentation law is used to calculate the contact force between the laminates and the rigid ball. To deal with the contact problem between delamination, we use an improved Lagrange multiplier method which was posed previously by the author. To explore delamination growth mechanism, we present a numerical method based on the Mindlin plate model to calculate the strain energy release rate at the front of a delamination. Finally, the influence of initial impact velocity, delamination area and delamination location is discussed. And the results of numerical examples provide a great deal of information for people to understand the delamination growth mechanism and its influence on the impact response of delaminated composite laminates.