The invention discloses an automatic locking method for a battery replacement station of a flexible direct-current transmission system. The automatic locking method includes the steps of firstly, judging the control mode of the station, the running state of the station and the running states of other stations; secondly, judging the current running state when the flexible direct-current system runs if the station is judged to have the direct-current voltage control mode, the station is in the unlocked state and meanwhile other stations are all in the locked state; thirdly, starting the locking process of the station according to a received system locking control instruction if all the conditions are met, and conducting reactive power reduction control; fourthly, sending a window signal to complete the locking process if the reactive power of the convertor station is zero at present. By means of the automatic locking method, a control and protection system is automatically locked, the locking process of the flexible direct-current transmission system is shortened, impact which is caused by control and protection system incorrect operation generated due to man-made incorrect operation and the like to an alternating-current system is effectively avoided, time consumption of the whole locking process of the flexible direct-current system is lowered, and the complexity in the locking process is lowered.