The invention relates to the technical field of bridge construction, and discloses a Bailey beam support dismantling device and a Bailey beam support dismantling method. The method comprises the following steps of 1, dismantling a full support above a Bailey beam, and dismantling the Bailey beam into a plurality of Bailey beam supports; 2, fixing a winch, and connecting a web plate of a cantilevercrane through a traction rope of the winch; 3, moving the cantilever crane to the position beside one Bailey beam support, hoisting the Bailey beam support by the cantilever crane, and starting the winch to pull the cantilever crane to a set position, thereby hoisting away the Bailey beam support; and 4, repeatedly carrying out the step 3 until all the Bailey beam supports are removed. By means of the device and the method, the problems of high noise, risk of unstability and falling of Bailey slices, and long construction time in the dismantling construction process can be effectively solved.