In order to improve the seismic reduction performance of the traditional tuned mass damper (TMD) to building structures, a novel tuned mass damper (TMD) named semi-active eddy current pendulum tuned mass damper (SAEC-PTMD) is proposed in this paper, which can retune its frequency and damping ratio in real time. The structural instantaneous frequency is identified through Hilbert-Huang transformation (HHT), and the pendulum of SAEC-PTMD is adjusted through HHT-based control algorithm. The relationship between effective eddy current damping coefficient and air gap was discussed and fitted, as well as the eddy current damping ratio could be adjusted in real time through adjusting the air gap based on the linear-quadratic-Gaussian (LQG)-based control algorithm. In order to verify the vibration control effect of the SAEC-PTMD, a single degree of freedom primary structure equipped with a SAEC-PTMD excited by earthquake excitations was proposed as a numerical simulation. An optimal passive TMD (PTMD) was used for comparison and the detuning effect was considered, which was caused by the cumulative damage of primary structure. Maximum and root-mean-square (RMS) value of structural acceleration and displacement time history response, structural acceleration and displacement response spectra were used as evaluation indexes to evaluate the improving effect of the SAEC-PTMD to PTMD, both before and after the damage happens to primary structure. The results show that SAEC-PTMD performs better than the optimized PTMD both before and after damage occurs to the primary structure and it has a better seismic reduction effect. [ABSTRACT FROM AUTHOR]