The performance of the steel scale (SS)-CaO composite on the combustion of coke nuts and in-situ reduction of SO2 and NO emission was investigated. Combustion experiments show that SS-CaO composite accelerates coke combustion and reduces SO2 and NO emissions concurrently. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron spin resonance (ESR) were used to investigate the catalysis mechanism. Results show that SS and CaO have synergistic effect in in-situ denitration. The increase of SS and CaO fraction in the composites improve the denitration and sulfur fixation performance, respectively. The denitration activity is mainly attributed to the Fe2O3 in SS and Ca2Fe2O5 formed during combustion. The desulfurization is due to the sulfur fixation with CaO. The increasing fraction of CaO in the composites leads to the agglomeration of ash particles. 2%CaO is confirmed as the optimum addition for catalytic combustion and emission reduction. This work sheds light on the cheap and effective catalysts that are potentially useful for iron ore sintering.