According to the existing problem that the restitution coefficient models ignoring constant external forces have the inaccurate energy dissipation results in the multibody dynamics, a new restitution coefficient model under a constant external force was presented.Hertz, Jackson and Green established a contact force and penetration depth relationship model, respectively, during the elastic and elastoplastic stages of the collision process.On the basis of the above, a new restitution coefficient model was established considering the constant external force and the principle of energy conservation during the collision process.In order to improve the calculation efficiency of the engineering practice, the calculation process was simplified and the empirical formula was obtained.By comparing with the existing restitution coefficient models, it can be concluded that constant external force has a great influence on the restitution coefficient at low speed and heavy load.Additionally, with the increase of constant external force, the penetration depth and plastic deformation increase, while the separation velocity decreases.According to the results, the new model can provide a more accurate calculation model in the multibody dynamics, with different initial velocities and constant external forces by considering the restitution coefficient under a constant external force.