This paper discusses the unified symmetry of a holonomic mechanical system in terms of quasi-coordinates under special infinitesimal transformations in which time is not varied. The differential equations of motion of the system are given. The definition and the criterion of the unified symmetry for the system are presented. A new conserved quantity, besides the Noether conserved quantity and the Hojman conserved quantity, is deduced from the unified symmetry. An example is iven to illustrate the application of the results.