Using an invariance of differential equations under the infinitesimal transformations of group, we study the Lie-Mei symmetry of Nielsen equations for a singular nonholonomic system of Chetaev'type. We establish the Lie-Mei symmetry equations. The definitions of weak and strong Lie-Mei symmetry are given, and Hojman conserved quantities and Mei conserved quantities are obtained. An example is given to illustrate the application of the results.