The utility model discloses a boiler including a water-cooling system used in a variable cross-section furnace chamber. The boiler including the water-cooling system used in the variable cross-section furnace chamber comprises a lower-portion furnace chamber and an upper-portion furnace chamber. The lower-portion furnace chamber is divided into a transition area, a lower furnace chamber and an ash hopper from top to bottom. Lower-portion front and back walls, side walls and upper-portion front and back walls are all membrane walls which are formed by combination of pipes and flat steel. The water-cooling system comprises spiral pipes and vertical pipes in one-to-one correspondence with the spiral pipes. All or parts of the membrane walls of the lower-portion furnace chamber are the spiral pipes. The water-cooling system in the boiler is simple in structure, flow quantity of working substances in the pipes is constant, and the problems that due to the fact that the flow quantity in one pipe is distributed to two or more pipes, mass flow rate in the pipes is reduced and then cooling effect of the spiral pipes and the corresponding vertical pipes is influenced are avoided.