Lie symmetry of Appell equation and conserved quantity deduced directly by Lie symmetry for a Chetaev's type constrained mechanical system are investigated. The relations between Lagrange function and A function are analyzed. A general approach of studying conserved quantity deduced by Lie symmetry of Appell equation for a Chetaev's type constrained mechanical system is discussed. The definition and the criterion of Lie symmetry of Appell equations under the infinitesimal transformations of groups are given. The structural equation of Lie symmetry and the expression of conserved quantity deduced directly by Lie symmetry are obtained. An example is given to illustrate the application of the results.