The Mei symmetry,i.e.the form invariance,of the Hamiltonian canonical equations in a singular system is studied.The definition,the determining equations,the restriction equations and the additional restriction equations of Mei symmetry of the system are given.The relations among the Mei symmetry,the Noether symmetry and the Lie symmetry are studied,and the conserved quantities of the singular system are obtained.An example is given to illustrate the application of the result.