The invention discloses a span change structure of a beam lifter. The beam lifter is a portal crane and comprises a main beam, a span change crown block and a connection bolt for the main beam and the span change crown block, wherein walking assemblies are arranged at the front and rear ends of the span change crown block; each walking assembly comprises a mobile idler wheel and a vertical jacking oil cylinder; the span change crown block is provided with a lifting lug to be detachably connected with a winch through a steel wire rope; at least four groups of walking assemblies are arranged at the front and rear ends of the span change crown block, and are fixedly connected with the span change crown block through connection plates; the jacking oil cylinders are fixed above the connection plates; the movable idler wheels are arranged below the connection plate and connected with the jacking oil cylinders through support shafts; the beam lifter is further provided with a guiding and positioning device. According to the span change structure of the beam lifter, the site conditions are reasonably utilized so that the span change is convenient; the original hydraulic system is utilized for controlling the vertical jacking oil cylinders at four corners so that the jacking and falling actions of the main beam are realized by controlling the stretching of the oil cylinder; the span change operation is realized through the traction of the steel wire rope of the winch. Therefore, the span change operation efficiency is well improved and the labor intensity is alleviated under the condition that the safety is ensured.