C6MIMBF4, C6MIMCF3COO and C6MIMHSO4 were introduced into a direct current discharge plasma system respectively for methane conversion. The kinds of active species and relative intensity of spectral peaks were detected via online spectrum diagnosis technique of optical emission spectroscopy. Mechanism of ionic liquid for methane conversion in gas-liquid plasma was investigated. The results showed that a spatially and temporally stable gas-liquid interface was obtained in ionic liquid incorporated plasma system. With introduction of ionic liquid, methane conversion and C2 hydrocarbons yields increased. The C2 hydrocarbon selectivity greatly increased when C6MIMCF3COO and C6MIMBF4 were introduced to the plasma, and decreased when C6MIMHSO4 was used. Active species like C, C2, C3, CH and H were detected in gas-liquid plasma system of methane discharge. Compared with the absence of ionic liquid in the plasma system, the relative intensity of spectral peaks of most active species increased when ionic liquid was introduced into the plasma system. 1H NMR results showed that the structure of ionic liquid kept stable during plasma discharge progress. Those indicated that ionic liquid could improve the plasma discharge intensity. At the same time, ionic liquid showed good catalytic activity in gas-liquid surface reaction.