The invention provides a big-belly tank type water electrolysis device and a preparation method thereof. The device comprises a big-belly bulb shaped glass container, wherein, an opening is formed at the upper part of the glass container and two electrodes are arranged at the lower part of the glass container; the two electrodes are connected with an electrolysis power source after extending out of the bottom of the glass container; the bottom of the glass container is sealed with the electrodes; and two gas collecting test tubes respectively buckled on the two electrodes are further arranged in the glass container. The main part of the water electrolysis device provided by the invention, a big-belly tank, is manufactured by completely utilizing a waste bulb which has extensive sources easy to obtain; and the waste is changed into valuables, so that the water electrolysis device is of great benefit to full utilization of the resources, environment protection and experiment cost reduction.