The invention relates to a method and a device for increasing acidic gas treatment capacity of acidic-gas sulfuric acid making device and belongs to the technical field of acidic gas treatment. A cracking furnace is provided with three spray guns, ammonia-containing acidic gas is connected with the main fuel gas spray gun, and the main fuel gas spray gun is connected with fuel gas; clean acidic gas is connected with the standby spray gun of the cracking furnace; the waste acid spray gun sprays waste acid into the cracking furnace. An air fan is provided with an air crossline for reducing furnace temperature. After the cracking furnace reaches the temperature higher than 1000 degrees Centigrade, the clean acidic gas is led into the furnace, feed of acidic gas flow into the waste acid cracking furnace is slowly increased while fuel gas usage of the waste acid cracking furnace is reduced; after the clean acidic gas flow is stable, the ammonia-containing acidic gas is imported. By the method and the device, acidic gas treatment capacity is increased by about 20-30%, SO2 gas concentration the conversion section is increased, furnace power is lowered, treatment cost is greatly reduced, flue temperature is increased and dew-point corrosion of an air preheater is avoided.