Sintering flue gas is the main source of air pollution in the iron and steel industry. Facing stringent emission standards set to limit the amount of flue gas pollutants, multi-pollutant control in sintering flue gas has become a necessity in the iron and steel industry. Utilizing biomass fuel with low S and N contents to replace a portion of coke in the sintering process can offer control at the source of sintering flue gas pollutants. This study is aimed at elucidating the reaction mechanism of mixed fuel in the sintering process using thermogravimetric experiments of the combustion and gasification reactions. The results show that the reaction performance of biomass fuel is better than that of coke. In addition, biomass fuel can improve the reaction performance of mixed fuel. When the content of biomass fuel increased from 20 to 80%, the temperatures of both the combustion and gasification reactions decreased, the weight loss rate of the mixed fuel increased, and the gap between the calculated and experimental values widened. By studying the reaction mechanism of mixed fuel, we found that the sintering fuel reactivity increases mainly because the porous structure of biomass fuel provides a large specific surface area for the combustion or gasification reaction, and the alkali metals, K and Na, in the biomass fuel act as catalysts to the reaction.