The optical emission spectrometry (OES) of methane ranging from 200 to 900 nm under plasma with different carbon dioxide contents were recorded and marked. Various reactive species in methane plasma such as: CHn(n=3， 2， 1)， H， C2， C， C+， CO2+， O， CO+， OH and CHO were detected in situ by OES with different CO2 contents at atmospheric pressure. The relative intensity of reactive species was different with different CO2 contents. The relative intensity of O reactive specie was increased rapidly and C2 reactive specie was reduced gradually with increasing CO2 contents. Reactive O from the dissociation of CO2 had an obvious influence on the methane conversion. The mechanism of methane conversion was changed when CO2 contents increased. The coupling of methane to C2 hydrocarbons was the main reaction when CO2 contents were smaller than 30%, while the reforming of methane played a dominant role when CO2 contents higher than 30%.