The present invention provides a dehydrogenation tank. An atomization spray is disposed at the center of the upper part of the dehydrogenation tank, and flow stirring modules used for stirring a flowing solution are respectively disposed at the middle and the bottom of the dehydrogenation tank. Each of the two flow stirring modules includes at least two layers of flow stirring meshes. By means of disposing the atomizing spray and the flow stirring modules in the dehydrogenation tank, the TRO solution flowing into the dehydrogenation tank are fully stirred, so that hydrogen gas mixed with the TRO solution is able to diffuse out fully and rapidly, thereby increasing dehydrogenation efficiency as well as reducing volume of the dehydrogenation tank. In addition, the present invention also provides a ballast water treatment system having the dehydrogenation tank.