A foldable device of golf bag cart comprises: an upper cart stand, a bottom cart stand and a linkage set. The upper cart stand, having an adapter that is provided with a slider on its top, is provided with a slider mount at each of the two distant ends above and below the slider. The bottom cart stand is in pinned connection with the adapter at its top. The linkage set, made up of a plurality of linkages, forms links between the slider and the bottom cart stand, between the adapter and the rear wheel frame and between the bottom cart stand and the front wheel frame. By means of the setup of such links, the simultaneous unfolding or folding of the upper cart stand, the bottom cart stand, the front wheel and the rear wheels can thus be accomplished, which facilitates the handiness in use.