The invention relates to an automatic fall-preventing device of a vertical carrier. The automatic fall-preventing device comprises a carrier and four stand columns matched with the carrier, wherein the inner sides of the four stand columns are respectively provided with guide rails, and the four stand columns are vertically arranged on the four corner side parts of the carrier; the carrier is inserted into the corresponding guide rails through a front beam and a rear beam fixed on the front and rear side parts of the carrier, and steel wire ropes arranged at the left and right sides of the carrier and a hoisting mechanism can form a carrier vertical lifting structure. The automatic fall-preventing device is characterized in that horizontal beam frames of the front beam and the rear beam are respectively provided with a connecting rod driving mechanism which can synchronously stretch out or shrink on the left and right horizontal directions, the left-right corresponding two groups of two guide rails are internally provided with brake blocks which are corresponding to the front beam and the rear beam; and thus, the vertical positioning structure of the automatic fall-preventing device of the vertical carrier is formed.