The invention provides a high-temperature denitration, fly ash re-ignition and in-furnace dust removal boiler. The high-temperature denitration, fly ash re-ignition and in-furnace dust removal boileris provided with a combustion chamber, a furnace and a long throat cavity connecting the combustion chamber with the furnace, and is further provided with a urea nozzle positioned adjacent to the junction of the long throat cavity and the furnace. The cross section of the long throat cavity is narrowed down relative to the combustion chamber and the furnace and the long throat cavity has a heightof 1-4 meters. In addition, the furnace is limited by a front wall, a left wall and a right wall of the boiler and a front screen in a specific shape, the front screen is approximately perpendicular to the left wall and the right wall and extends from the left wall to the right wall, the upper end of the front screen is connected to a boiler drum in the top wall of the boiler, and the lower end ofthe front screen is connected to a lower header of the front screen; and the part, limiting the furnace, of the front screen forms a concave surface facing the inner recess part of the front wall, and thus a travel path of flue gas is extended. In addition, the boiler includes a horizontal flue which communicates with the top of the furnace, the boiler is provided with a trough type separator located in the horizontal flue, a flue-dust retainer is arranged below the trough type separator, and the flue-dust retainer can selectively communicate with the furnace, so that collected flue dust is fed into the furnace for re-ignition.