A method for modal parameter identification of time-varying structures based on Hilbert- Huang transform (HHT) is presented. Theoretical formulas for identifying the modal frequency and damping radio using the displacement response of a time-varying SDOF structure are deduced. Taking advantage of modal filtering characteristics of empirical mode decomposition (EMD), the presented method is expanded to identify the modal parameters of MDOF structures. Numerical simulation of a three degrees of freedom structure with time-varying stiffness and damping show the validity of the method. Finally, a time-varying structure experiment is designed to further study the method. The experimental device is a cantilever beam. By adjusting the adjunctive mass and stiffness, two kinds of time-varying structures with continuous mass change and stiffness change is realized respectively. Modal parameters are identified from the free acceleration response of the beam.