The invention discloses a steel tube air preheater which comprises tube sets, an air inlet pipeline and an air outlet pipeline. The tube sets at the heating surface of the air preheater are arranged in a 'U' shape; the tube sets and an air tube are welded on the air tube after penetrating through an enclosed wall; the tube sets and a fixing steel plate are fixed through welding; the tube sets and the enclosed wall are sealed through a seal cover which is welded on the enclosed wall; the tube sets and a comb-shaped plate are welded; the comb-shaped plate and a supporting plate are welded; and the supporting plate is welded on a water cooling enclosed wall. The air preheater provided by the invention is designed resonably, has a simple structure, has high abrasion-resistant capability, and has obvious heat-resisting and heat exchange effects, through the heating of the preheater, the primary heat air temperature is effectively improved to more than 400 DEG C, the low-heating-value waste is combusted fully, and the combustion efficiency of a waste combustion boiler is improved.