The invention provides a preheater. The preheater comprises two hollow flanges, four vertical columns, two tube plates, two side plates, the two sealing plates and a pipeline, the vertical columns areconnected with the hollow flanges in a welding mode, the tube plates and the side plates are arranged alternately and are arranged between the two hollow flanges, the tube plates is connected with the vertical columns in a welding mode and is connected with the hollow flanges in a welding mode, the tube plates are provided with tube plate grooves with openings facing the outside of the preheater,first through holes are formed in the tube plate grooves, the side plates are connected with the vertical columns in a welding mode and are connected with the hollow flanges in a welding mode; the sealing plates are provided with second through holes corresponding to the first through holes, the two sealing plates are welded and connected with openings respectively, the pipeline is arranged in the first through holes and the second through holes in a penetrating mode, the two ends of the pipeline are connected with the second through holes of the sealing plates in a welding mode, and the welding connection is in full-welding connection. Therefore, the preheater can prevent the heat source in the preheater from leaking from joints between the hollow flanges, the vertical columns, the tubeplates, the side plates and the pipeline to the outside of the preheater, so that the heat exchange effect is good.