Subspace-based methods for estimation of modal parameters are briefly reviewed in this study and a time-varying modal parameter identification algorithm, based on finite-data-window Projection Approximation Subspace Tracking, is presented to investigate the time-varying modal parameters of a trapezoidal titanium-alloy plate in temperature-varying environments. An experiment conducted on a steel beam with a removable mass is used to confirm the proposed method with a brief discussion on the factors of this method. Two groups of experiments are conducted to reveal the effects of varying temperature and heating speed on the natural frequencies of the plate, and the identified natural frequencies evidently show the effect of thermal stresses caused by temperature gradients in experiment.