Abstract: Time-varying modal identification is important to obtain the dynamic characteristics of time-varying engineering structures and realize real-time vibration control and online health monitoring of structural systems. Aiming at the problems of measuring the excitation of engineering structures and low efficiency of time-varying modal identification， an output-only recursive identification method based on dynamic mode decomposition （DMD） is proposed in this paper. To extend the theory of the existing DMD method， this paper draws on the projection approximation subspace tracking algorithm and the sliding-window idea， and proposes a recursive format of the DMD method， which can update the system matrix and the proper orthogonal decomposition basis recursively， and can be further used for the output-only recursive modal identification of time-varying systems. A numerical example of a three-degree-of-freedom structural system with time-varying mass and an experimental setup of a liquid-filled cylindrical structural system with variable mass are respectively designed and built to validate the proposed method numerically and experimentally. The results demonstrate that the proposed recursive DMD method is able to accurately identify the modal parameters of the time-varying structures by only using the measured vibration response data， and has good output-only recursive identification capability.