The invention discloses a device and method for mounting and dismounting a crane boom. The device comprises a main chord member, a transverse steel girder, a longitudinal steel girder, an amplitude-variable cart, a derrick crane, a movable pulley set, an amplitude-variable steel wire rope and a lifting steel wire rope, wherein the derrick crane is fixed to a derrick crane base; the lower ends of the transverse steel girder, the longitudinal steel girder and the main chord member jointly define a semi-closed rectangular area; the amplitude-variable cart is clamped in the semi-closed rectangular area; the amplitude-variable steel wire rope is connected with a chain block, one end of the amplitude-variable steel wire rope is fixed to the derrick crane, and the other end of the amplitude-variable steel wire rope is fixed to the main chord member; one end of the lifting steel wire rope is fixed to the main chord member, and the other end of the lifting steel wire rope is wound around the movable pulley set and then is fixed to the derrick crane. According to the device and method for mounting and dismounting the crane boom, the amplitude-variable cart is matched with the derrick crane and the movable pulley set, the crane boom is mounted and dismounted, the space and advantages of the amplitude-variable cart are fully used, the crane boom can be hoisted to be disconnected from a tower crane at a time, the mounting and dismounting structure is simple, operation is simple, and feasibility is high.