Abstract: Mei conserved quantity led by Mei symmetry of a Nielsen equation for a holonomic system in terms of quasi-coordinates was discussed.The definition and the criterion of Mei symmetry of a Nielsen equation for a holonomic mechanical system in terms of quasi-coordinates are given,and the condition and the form of the Mei conserved quantity deduced directly from the Mei symmetry of the Nielsen equation for a holonomic mechanical system are discussed.Finally,an example is given to illustratethe application of the results.