Automatic leg tube locking device, photography tripod, unipod and crutch

An automatic leg tube locking device, comprising a guide switch and an guide post which are mutually cooperative in use, wherein the guide switch is a hollow cylinder without a bottom surface, the guide post is in a circular truncated cone shape, a plurality of through holes are evenly formed in the side surface of the guide switch, the through holes are provided with locking shafts, a transmission mechanism is arranged between the guide switch and the guide post, so that the guide switch vertically moves along the inclined cylinder surface of the guide post and drives the locking shafts to move along the guide post when the guide switch and the guide post rotate relative to each other, the upper surface of the guide post being provided with an inclined guide surface, and the inner bottom surface of the guide switch being provided with a mating surface that is mutually conflicting to the inclined guide surface.