Abstract: A locking device of extendable and retractable container semi-trailers comprises four link bars, two lock pin assemblies, a crossmember, a falling tube and a pull assembly. Two ends of the pull assembly are connected with two link bars and one lock pin assembly respectively. Said pull assembly comprises an automatic locking and releasing device fixed rotatablely between a bottom plate and a top plate of said crossmember. The pull assembly is rotated round the automatic locking and releasing device to a locking place, and the automatic locking and releasing device locks said pull assembly. The falling tube acts on the automatic locking and releasing device, and the pull assembly is released.