A unified power flow control er (UPFC) combines the advantages of various flexible altating current transmission system (FACTS) devices into a powerful format. Using a 500 kV power grid, this study evaluates the selection and use of a UPFC to improve transmission capacity. The UPFC unit capacity control proportionality coefficient is introduced to quantify the control effect of the UPFC, and an optimal calculation method for the UPFC capacity is presented. Following the proposal of a UPFC site selection process, the data of an existing power grid is used to conduct simulations. The simulation results show that the UPFC has a strong ability to improve transmission capacity, and its use is greatly advantageous. Additionally, by applying the proposed selection method, the control effect and economic benefits of the UPFC can be comprehensively considered during project site selection. These findings have a guiding significance for UPFC site selection in ultra-high voltage power grids.