The invention relates to a control device and method for putting a converter station into a flexible direct current power transmission system. The device and method is characterized in that the existing converter station isolation switch is replaced with a control device. The control device comprises a direct current circuit breaker, two direct current voltage dividers and a voltage comparison controller. The method further includes closing the direct current circuit breaker in the control device of an off-line or overhauling converter station, starting a converter in the off-line or overhauling converter station, comparing voltage signals on the off-line or overhauling converter station side or on the operating converter station side and obtained by the two direct current voltage dividers through the voltage comparison controller in the control device in an operating converter station to judge whether direct current voltage at two ends of the direct current circuit breaker in the operating converter station is identical, enabling output signals of the voltage comparison controller to control direct current voltage at two ends of the direct current circuit breaker to be identical, closing the direct current circuit breaker and putting the off-line or overhauling converter station into the system. The device and method can be widely applied to the flexible direct current transmission system.