±800 kV Yun-Guang DC transmission project is designed with two series 400 kV converters for each pole，and each converter is designed with an independent last circuit breaker protection. The principle of last circuit breaker protection in inverter station in Yun-Guang ultra high voltage direct current（UHVDC）project is analyzed in detail. It is pointed out that the protection logic of last breaker protection exists major risk to power grid. An improved scheme of the tripping logic is given and executed on site to avoid the risk，which has important reference value on the design of high voltage DC transmission system and stable operation of power grid.