Type Ⅲ structural equation and Mei conserved quantity of Mei symmetry for a Lagrangian system are studied. Under the infinitesimal transformation of groups, type Ⅲ structural equation and Mei conserved quantity of Mei symmetry for a Lagrangian system are obtained from the definition and the criterion of Mei symmetry for a Lagrangian system. Finally, an example is given to illustrate the application of the results.