This paper studies the Mei symmetry and Mei conserved quantity of non-Chetaev's type in the event space. The differential equations of motion of non-holonomic s ystems of non-Chetaev's type in event spaces are established. The definition and the criteria of Mei symmetry are given. The condition and the form of Mei conse rved quantity are deduced directly from the Mei symmetry. An example is given to illustrate the application of the results.