Abstract: To study the gene diversity involved in the Kalvin pathway of carbon fixation and its response to environmental variables in the lake surface sediments on the northern Qinghai- Tibetan plateau. In this study, we collected surface sediments from six typical lakes (Erhai Lake, Qinghai Lake, Tuosu Lake, Gahai Lake, Xiaochaidan Lake, Chaka Lake) on the Qinghai- Tibetan plateau. The carbon- fixing bacteria diversity in the collected samples was investigated by using the rbcL gene- based phylogenetic analysis. Correlation analysis was performed between the rbcL gene diversity and environmental parameters of the studied lakes. The phylogenetic analyses show that the rbcL gene population in the studied samples are classified into three phyla, i.e. Proteobacteria , Cyanobacteria , and Chlorophyta , and into eight classes, i.e. Acidithiobacillia , $\alpha$- Proteobacteria , $\beta$- Proteobacteria , $\gamma$- Proteobacteria , Chroococcidiopsidales , Oscillatoriophycideae , Synechococcales , and Chlorophyceae . In the freshwater lakes (Erhai Lake), the Synechococcales - related clones are dominant (the relative abundance is 71.32%); In the saline lakes (Qinghai Lake, Tuosu Lake), $\beta$- Proteobacteria , Synechococcale , and Oscillatoriophycideae are dominant classes (the relative abundances are 36.20%, 23.47%, and 22.02%, respectively); In the hypersaline lakes (Gahai Lake, Xiaochaidan Lake, Chaka lake), Acidithiobacillia and Chlorophyceae ~are dominant classes (the relative abundances are 53.33% and 30.40%, respectively). The rbcL gene population composition show significant ( p 0.05) correlations with salinity, dissolved organic carbon (DOC), TP, TN, dissolved inorganic carbon (DIC), pH and chlorophyll- a. Aggregated boosted tree (ABT) analyses show that TP and salinity are the two factors contributing the most (accounting for 20.04% and 16.81% of the microbial variation) to shaping carbon fixing microbial communities in the studied lakes. The majority of the rbcL gene population is affiliated with the Proteobacteria (mainly Acidithiobacillia ) and Synechococcale in the studied lake surface sediments. The rbcL gene population composition varies significantly among the studied lakes of different salinities, and the lacustrine habitats with similar salinities possess similar rbcL gene population. In summary, TP and salinity is an important factor affecting the distribution of carbon- fixing bacterial~population.