The unified power flow controller can improve the transmission capacity of the transmission line and enhance the system stability level. However, when it is connected to the grid, the voltage, current and impedance in the line will change, which may cause malfunction of the line relay protection. The working principle of the unified power flow controller is introduced. The effects of the unified power flow controller and the static synchronous compensator and the static synchronous series compensator on the line relay protection (including distance protection and longitudinal protection) are summarized. The existing solutions and their deficiencies are analyzed, and corresponding suggestions are given for the future research. This work is supported by National Key Research and Development Program of China (No. 2016YF0900604).