Mei symmetry and Mei conserved quantity for a system with unilateral non-Chetaev nonholonomic constraints in the event space are studied. The differential equations of motion of the system are established. The definition and the criteria of Mei symmetry, weakly Mei symmetry, strongly Mei symmetry for the system are given in this paper. The existence condition and the expression of Mei conserved quantity are deduced directly from Mei symmetry. An example is given to illustrate the application of the results.