Rotor systems applied widely in industry are nonlinear dynamic systems of multi-degree of freedom. They must be not only stability but also have enough stability margins. Stability margins for different motion states in rotor systems are defined based on trajectory-based stability-preserving dimensional-reduction (TSPDR) in a quantitative stability analysis method for rotor systems. With the help of stability margin, dynamic behavior analyses of rotor systems become easy. In this paper, experimental method is used to validate definition and law of stability margin of quasi-periodic motion which was presented in above method. Experiment results in a rotor experiment rig show that regular pattern of stability margin of quasi-periodic motion calculated from measured data accords with the rule of above method. Therefore the definition and the law of stability margin of quasi-periodic motion described in the quantitative stability analysis method are valid in real rotor systems.