The Mei symmetry and the Mei conserved quantity of Appell equations in a dynamical system of relative motion with non-Chetaev nonholonomic constraints are studied. The differential equations of motion of the Appell equation for the system, the definition and the criterion of the Mei symmetry, and the expression of the Mei conserved quantity deduced directly from the Mei symmetry for the system are obtained. An example is given to illustrate the application of the results.