The invention relates to a 40 thousand ton/year soft carbon black reaction furnace for producing N774 carbon black. A secondary air distributor is divided into two step cavities. Air pipelines are separately arranged on the upper and lower sides of the two step cavities to be tangentially connected to the step cavities to form an air path, and ventilating slots are formed in the connections of theair pipelines and the step cavities. The technical scheme of the invention is as follows: N774 carbon black is produced by the 40 thousand ton/year soft carbon black reaction furnace, the output is enhanced, the methylbenzene light transmittance is improved, the water washing residue on sieve is reduced, and the quality of the N774 carbon black is improved. The combustion effect of a combustion room is improved, the reaction temperature is ensured, the atomizing effect of raw material oil is improved and the consumption of the raw material oil is reduced.