The invention provides a slab segment and a slab segment replacement system. The slab segment replacement system comprises an overhead crane, wherein the lower end of the overhead crane is connected with lifting appliances, and a guide rail is fixed to the lower end of a casting table. The lifting appliances include a long lifting appliance and a short lifting appliance, the two sides of the long lifting appliance and the two sides of the short lifting appliance are provided with first guide wheels and second guide wheels respectively, and the first guide wheels and the second guide wheels are symmetrical, are arranged evenly and perform reciprocating motion on the guide rail. A first pin hole penetrating through the lateral sides of the replacement guide rail is formed in the guide rail, and a first hinge pin matched with the pin hole is arranged in the pin hole. The slab segment replacement system is composed of the overhead crane, the long lifting appliance, the short lifting appliance and the guide rail, the long lifting appliance and the short lifting appliance are used at different times when the slab segment replacement system is used, and the space distance required by replacement of the slab segment is shortened. The distance between the casting table and the upper limit of a hook of the overhead crane is shortened, so that investment is reduced, and resources are saved. In addition, slab knockout difficulty of a large new-built slab caster in an old factory building is reduced.