The invention relates to a method for utilizing heat energy of flue gas produced by calcining in a rotary kiln in the process of producing titanium dioxide by a sulfuric acid method, which comprises the following three aspects of: (1) performing electric precipitation to purify high-temperature flue gas produced in a rotary calcining kiln, collecting dust by a dry method, and returning a recycledproduct to the rotary kiln for calcining again; (2) spraying 20 percent waste acid produced in the production process to one part of the purified high-temperature flue gas, performing mass transfer and heat transfer, evaporating moisture in the waste acid to improve the concentration of the waste acid to 30 percent, purifying 70 to 80 DEG C low-temperature tail gas produced by concentrating the waste acid by using an electric demisting system, and discharging the tail gas when the tail gas reaches the standard; and (3) returning the other part of the purified high-temperature flue gas to the rotary calcining kiln for recycling, supplementing heat, and adjusting temperature gradient in the kiln. Materials in the flue gas are recycled by the dry method, and the yield of a product is improved by about 0.6 percent; and a large amount of flue gas waste heat is comprehensively utilized, and the effects of saving energy and reducing emission are obvious. Meanwhile, the invention discloses a set of device for utilizing the heat energy of the flue gas produced by calcining in the rotary kiln, which is designed according to the method.