The utility model discloses a four-megaton hard carbon black reaction furnace. A furnace body comprises a combustion chamber, a throat pipe, a reaction chamber and a quench zone which are connected in sequence; the combustion chamber comprises a straight pipe section and a horn section, wherein the straight pipe section occupies two thirds to three fourths length of the combustion chamber and is provided with a fuel inlet; the throat pipe comprises a horn section and a straight pipe section, a taper angle of the horn section is 9-15 degrees, the caliber of an opening end of the horn section is 1.7-2.0 times that of a shrink end of the horn section, the inner diameter of the straight pipe section is consistent with the caliber of the shrink end of the horn section, and the straight pipe section is provided with two rows of raw oil nozzles; and the reaction chamber is horn-shaped, a shrink end of the reaction chamber is connected with the straight pipe section of the throat pipe, an opening end of the reaction chamber is connected with the quench zone, the caliber ratio of the shrink end and the opening end of the reaction chamber is 1:1.8-1:2.3; and the quench zone is in a two-section type, the inner diameter ratio of the two sections is 1:2-1:2.5. The four-megaton hard carbon black reaction furnace adopts gas or natural gas as fuels, is wider in range of raw oil, high in carbon black productivity, low in single consumption, less in emission and high in carbon black quality.