The invention provides a sectional tube nest cooling device in a decomposing tank. The sectional tube nest cooling device is composed of multiple sections of cooling tube nests arranged in the decomposing tank, each decomposing tank is divided into multiple sections of heat exchange area from top to bottom, a section of cooling tube nest is arranged in each heat exchange area, and all sections ofcooling tube nests in the same decomposing tank are in parallel connection; each section of cooling tube nest is constituted by an upper annular pipe, a lower annular pipe and a tube nest cluster between the upper annular pipe and lower annular pipe, and the tube nest cluster is constituted by several vertical pipes parallel to the axis of the decomposing tank; water inlets of all sections of cooling tube nests are all communicated with a water inlet main pipe, and water outlets of all sections of cooling tube nests are all communicated with a water return main pipe. Decomposing slurry is directly cooled through sectional tube nest cooling devices in the decomposing tanks, the cooling procedure outside the tank is replaced, equipment investment and running cost are saved, and meanwhile, the work of the operating amount of workers and daily maintenance is reduced.