Conformal invariance and conserved quantity for a variable mass holonomic system in relative motion have been studied. The definition and the determining equations of conformal invariance for a variable mass holonomic system in relative motion are given. The necessary and sufficient conditions that the system’s conformal invariance be of Lie symmetry are deduced. With the aid of a structure equation which the gauge function should satisfy, the system’s corresponding conserved quantity is obtained. Finally, an illustrative example is given to verify the results.