The utility model discloses a step-by-step reducing phase change heat accumulator which comprises a heat accumulator shell, a plurality of heat exchange tubes arranged in the heat accumulator shell and a phase change heat accumulation material filled in the heat accumulator shell. The heat exchange tubes are parallel to one another and are arranged in the horizontal direction, the heat exchange tubes are arranged in multiple layers from bottom to top, and each layer of heat exchange tubes are located in the same horizontal plane; the pipe diameters of the heat exchange pipes on the same layerare the same and are increased layer by layer; each heat exchange pipe located on the lower layer is communicated with the adjacent heat exchange pipe located on the upper layer through a U-shaped pipe in a sealed mode to form an S-shaped communicating pipe. The U-shaped pipe is arranged outside the heat accumulator shell, and a plurality of through holes are formed in the side wall of the heat accumulator shell so that two openings of the U-shaped pipe can penetrate through the through holes to be communicated with the heat exchange pipe in a sealed mode. The heat storage medium can keep energy distribution balance in the heat storage process, the problems of uneven thermal stress and deformation caused by uneven heat distribution are solved, the service life of equipment is prolonged, and the heat storage effect of the equipment is improved.