The Mei symmetry, i.e.the form invariance, of a Vacco system is studied. The Definition and the determining equation of Mei symmetry in the Vacco system are given. The relations among the Mei symmetry, the Lie symmetry and the Noether symmetry are studied, and the conserved quantities of the Vacco system are obtained. An example is finally given to illustrate the application of the result of this paper.