In the feed water circulating system, heat exchanger is used to replace the boiler, and waste heat of flue gas is produced by flue-cured tube oven. The water is heated from 60 to 65 degrees Celsius. Through the retrofit of waste heat recovery from flue gas, 60 kW of heat was recovered. Each tube oven saves about 6 Nm3/h of natural gas. The annual economic benefit is about 80,000 yuan.