In this paper, the co-combustion characteristics and coupling mechanism of coal gasification fly ash (CGFA) and coal gangue (CG) were studied by Equivalent Characteristic Spectrum Analysis (ECSA®). ECSA® provided a quantitative analysis of gaseous products during the combustion process coupling the TG-MS system. Based on the mass flow rates of O2 consumption and gaseous products, we proposed a novel method of analyzing the co-combustion coupling process. The instantaneous co-combustion process was divided into CGFA and CG corresponding to an independent combustion process. In addition, the proportion of components participating in the co-combustion was calculated, which could reveal the coupling mechanism. The results indicated that the combustion intensity of CGFA was better than that of CG. As the mass ratio of CGFA increased, the value of the combustion characteristic index gradually increased. Furthermore, the ignition coupling interval and the co-combustion coupling interval existed during the co-combustion process, which was caused by the coupling oxidation of carbon. The nitrogen-containing gaseous products mainly included NH3 and NO. The coupling release of NH3 was derived from the conversion of char-nitrogen. Given the co-combustion characteristics and released gaseous products of mixtures, the mass ratio of CGFA and CG at 2:1 was suggested in actual co-combustion application.