In this paper, we propose a method for identifying the instantaneous modal parameters of a time-varying structure based on a continuous wavelet transform. First, we selected a random impact load as the input excitation of a time-varying structure. Then, under the assumed time-varying condition in a short time span, we established a principle for identifying the time-varying parameter based on the continuous wavelet transform. We identified the instantaneous modal parameter using only the output response of the structure, and performed a numerical simulation of a 3-DOF time-varying structure system. The results show that this method can accurately identify the instantaneous modal parameter of a time-varying structure. Finally, we designed and tested a two-layer steel frame model with variable quality parameters, and the results further verified the validity and feasibility of the proposed method.