In light of the influence of aero-engine bearing assembly conditions on support stiffness and provide support for dynamic design of aero-engine rotor system and wholeengine vibration control, the identification of the support stiffness by the vibration test data and theoretical calculation is derived based on kineticequations. Experiments are carried out to investigate the variation laws of the support radial stiffness which are changed with the inner and outer ring fit tolerance and the inner and outer ring lock nuts tightening torque of an aero-engine No.5 bearing. The results show that the identification method is correct and viable. When the inner or outer ring is interference fit, the magnitude of interference increases as the support stiffness increases. The support stiffness is insensitive to the inner and outer ring lock nuts tightening torque. When the inner or outer ring is clearance fit, nonlinear resonance appears on the rotor, the bearing stiffness fluctuates randomly and significantly, and is incompatible to the assumption of linear conditions. In short, the fit parameters of inner and outer rings should be selected reasonably in design, in order to avoid deterioration of rotor system vibration features.