Due to the problem that the tuned mass damper (TMD) system is easy to be off-tuned when applied to light structures, which leads to the decline of vibration reduction effect, a new shape memory alloy semi-active TMD system is designed in this paper. The system uses steel cables to suspend the mass to bear all its weight. Large shape memory alloy bars with rectangular effective cross sections are used to provide different bending stiffness in the two directions of the TMD system in the horizontal plane. In order to study the semi-active performance of the system, a full-scale shape memory alloy TMD system was subjected to free vibration tests in this paper. By changing the working temperature of the shape memory alloy, the influence of temperature change on the frequency and damping ratio of the TMD system was studied. The test results show that by controlling the working temperature of the shape memory alloy from -40 ℃ to 80 ℃, the frequency of the TMD system shows an increasing trend with temperature increases, while the damping ratio shows a decreasing trend with the temperature increases. The results show that the new shape memory alloy semi-active TMD system is applied to controlled structures, and once the TMD is off-tuned, it can be re-tuned by changing the temperature of the shape memory alloy. Therefore, the new shape memory alloy TMD system designed in this paper has certain engineering application value and prospect for the study of light structure vibration reduction.