Power frequency and transmission line current show a distinct process of fluctuation, transition, and recovery when massive active power imbalance occurs, which is caused by internal and external disturbances. Thus, attention should be paid to the thermal safety of conductor, and reasonable protection strategy should be adopted during the recovery of power system. In this paper, the dynamic characteristics of power frequency and line current are analyzed. Furthermore, the influence of different UFLS schemes on the frequency recovery rate and conductor temperature rise response is also studied. Thermal overload protection of transmission lines considering the influence of UFLS under active power imbalance is proposed. The effectiveness of the proposed protection strategy is verified on IEEE 39-bus power system. The simulation results show that different location, number and setting scheme of UFLS have different influence on conductor temperature rise response and time delay of protection under different disturbances. The proposed overload thermal protection can maximize the capacity of the transmission lines to withstand emergency overload, and avoid new cascading fault due to conductor temperature off-limit when the power frequency is recovery. This work is supported by the Youth Science Fund Project of National Natural Science Foundation of China (No. 51707018), National Key Research and Development Program of China (No. 2016YFB0900600), and Chongqing Postdoctoral Research Project Special Funding (No. Xm2017106).