Salinity, which induces changes in the soil microbial function and metabolic pathways, has important implications for soil carbon cycling. We carried out 13CO2 labelling experiments and metagenomic sequencing analysis for soil samples from Zhalong, Momoge, Niuxintaobao, and Xianghai sites in Songnen Plain. We found that the CO2 fixation rate of soil microbe in Songnen Plain wetlands ranged from 39.78 to 147.22 mg C m-2 day-1 in the natural salinity condition. Soil salinity increasing 1% and 2% decreased the microbial CO2 fixation amount. Elevated soil salinity increased soil microbial diversity (Chao 1 index) and altered its composition. Soil carbon fixing microbial abundances (Sphingomonadaceae and Lysobacter) decreased with salinity increase and were positively correlated with soil microbial CO2 fixing amount. The abundances of salt resistant microbes (Rhodohalobacter, Nitriliruptor, and Halomonas) increased with soil salinity increase, but were negatively correlated with soil microbial CO2 fixing amount. The rTCA cycle, 3-HP cycle, DC/4-HB and Calvin cycles were the predominant soil carbon fixing pathways under the natural salinity condition. Soil microbial abundances involved in rTCA cycle in Xianghai, Niuxintaobao wetlands and 3-HP, DC/4-HB cycles in Zhalong, Momoge wetlands decreased with salinity increase. 3-HP cycle was the predominant soil carbon fixing pathway in the 1% salinity addition treatment. rTCA cycle and Calvin cycle were the predominant soil carbon fixing pathways in the 2% salinity addition treatment. These findings contribute to a holistic view of soil microbial communities and carbon fixation functions in response to soil salinity, and provide new insights into carbon sequestration and carbon management in wetlands.