With the increasing complexity of the power system, more and more regional power grids are connected with each other. Hence, how to improve the stability and reliability of the power system is in urgent need to be solved. Low frequency oscillation, generated from the connection of different grids, brings high challenge to the safe and stable operation of the power grids. In this paper, the collaborative control strategy of large-scale flexible load and Unified Power Flow Controller (UPFC) is studied to suppress the low-frequency oscillation of the power system. The control strategy for large-scale flexible load based on frequency feedback is established, and the additional control strategy based on active-power feedback is designed at the UPFC series side. Finally, genetic algorithm is adopted to optimize the parameters of the cooperative control strategy, so as to develop an optimal cooperative control strategy. The proposed method is validated by numerous examples. This work is supported by National Natural Science Foundation of China (No. 51707099).