Reverse water-gas shift (RWDS) 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 properly adjusting the pressure of reaction system, it is possible to suppress the occurrence of RWGS reaction and increase the selectivity of CH4/CO2 reforming. The intermediates, such as CHO and CH3 of CH4/CO2 reforming was observed by means of liquid-N2 for quenching technique and primarily confirmed a reaction step: CH3+ [O] = CHxO. In addition, XPS results verified that nickel over catalyst after CH4/CO2 reforming existed in elemental state. C1s and O1s spectra of catalyst observed before and after reforming indicated the adsorption of CO2 species reduced in working La2NiO4 catalyst, possibly related to the desorption of CO2 under the condition of vacuum.