Unified Power Flow Controller (UPFC) integrates a variety of control strategies, which can complete multiple operating conditions in sequence, and the smooth switching between multiple control strategies is important for the stable operation of UPFC. A trigger control logic suitable for free switching between multiple control strategies is proposed, which can realize the smooth handoff between UPFC multiple control strategies and ensure the stable operation of power transmission system combined with the soft switching logic of each control strategy. The simulation system of UPFC is built on the RTDS platform, and simulation tests of smooth switching between different control strategies of UPFC system are designed for testing the proposed control logic. The experimental results show that the improved control algorithm is effective and feasible. This work is supported by National Natural Science Foundation of China (No. 51467003).