A movable side wall and opening/closing mechanism for railroad boxcars, comprising a side wall assembly (1), an end-portion opening/closing mechanism (2), a hinged rod assembly (3), a lateral door central clamping device (4), a upper transmission shaft assembly (5), a lower transmission shaft assembly (6), a upper track assembly (7) and a lower track assembly (8); upon loading/unloading cargo, the movable side wall can be opened further, so as to allow multiple forklifts of any tonnage on the ground to load/unload cargo to/from any positions of a carriage; the hinged rod assembly (3) and the central clamping device (4) are provided to keep the movable side wall reliably closed in order to ensure security and self-locking during transportation.