To investigate the effects of plant species and diversity on methane emissions and functional gene abundances under middle influent C/N ratio, we developed laboratory-scale constructed wetlands. Two plants, Lythrum salicaria and Pontederia cordata, which are often used in constructed wetlands and have good landscape values, were selected to establish current planting patterns, e.g. the one-species monoculturing and two-species mix culturing systems. Results showed that methane emissions from the mix culturing systems were higher (8.78 mg CH₄ m^-2 d^-1) than the mean values of the two monoculturing systems (6.97 mg CH₄ m^-2 d^-1) (P < 0.001). Similar to methane emissions, the absolute abundance of mcrA from mixed systems was also higher (977541.6 copies/g dw soil) than the mean value of the two monoculturing systems (585146.8 copies/g dw soil). However, the absolute abundance of pmoA from the mixed systems (326956.6 copies/g dw soil) was lower than the mean values of the two monoculturing systems (1043616.0 copies/g dw soil) (P < 0.001). In addition, both microbial biomass and plant biomass in the mix culturing systems were higher (P < 0.01), but the NH₄⁺-N concentration of effluent in the mix culturing systems was lower than the mean value of the two monoculturing systems (P < 0.05), and there was no significant difference in TOC and NO₃^--N concentrations between the two systems (P > 0.05). There was a significant difference in the methane emission, absolute abundance of pmoA, microbial biomass, plant biomass, and effluent NH₄⁺-N concentration between two monoculturing systems planting with Lythrum salicaria and Pontederia cordata, but there was no significant difference in the absolute abundance of mcrA, effluent TOC and NO₃^--N concentrations between the two monoculturing systems with Lythrum salicaria and Pontederia cordata. In order to achieve the high purification efficiency of constructed wetlands, it is necessary to mix planting of Lythrum salicaria and Pontederia cordata, but mix planting enhances methane release. By using the variation (ω²), we found that although the effect of plant species on methane emissions, pmoA absolute abundance and effluent NH₄⁺-N concentration was greater than that of plant species richness, the effect of plant species on mcrA absolute abundance was smaller than the effect of plant richness.