Abstract: The present disclosure provides a real-time identification method for a nonlinear characteristic of a model-free in-service seismic isolation/vibration reduction device. The method includes: dividing a structure of a seismic isolation/vibration reduction system into a plurality of substructures, and defining a substructure where the seismic isolation/vibration reduction device is located as a target substructure; and using a General Extended Kalman filter with unknown inputs (GEKF-UI) to identify a linear stiffness and a damping coefficient of the seismic isolation/vibration reduction device and a linear stiffness and a damping coefficient of the target substructure, in the case that an external excitation on the seismic isolation/vibration reduction device is small so that the seismic isolation/vibration reduction system is in a linear state.