Reverse water-gas shift (RWGS) reaction over La2NiO4 catalyst was investigated during CH4/CO2 reforming by continuous wave cavity enhanced absorption spectroscopy, a sensitive absorption technique. The results indicated that the RWGS reaction promoted the conversion of CO2 and decreased the partial pressure of hydrogen. By proper adjustment of the pressure of the reaction system, it is possible to suppress the occurrence of RWGS reaction and increase the selectivity of CH4/CO2 reforming. The intermediates of CH4/CO2 reforming such as CHO, OH and CH3 were observed by means of the liquid-N-2 quenching technique, and a reaction step: CHx + [O] = CHxO was primarily confirmed. In addition, XPS and XRD results verified that nickel over catalyst after CH4/CO2 reforming existed in elemental state. The HRTEM images of used catalyst indicated that the deactivation of catalyst was originated mainly from the formation of nanotubes and graphite carbon on active sites of nickel.