A microcosm experiment was conducted using litter bag method to explore the changes of rice straw decomposition rate under different amounts of nitrogen-fixing cyanobacteria addition, in which four treatments were designed, i.e., straw addition (CK), straw addition with low, medium and high amounts of cyanobacteria addition (LBA, MBA and HBA), and Biology method was used to determined carbon source metabolic activity potential (AWCD) and αdiversity of straw decomposing microorganisms. All treatments were incubated at 10 ℃ for 90 days, straw samples were collected after 30, 60, and 90 days to analyze straw decomposition rate, AWCD and αdiversity. The results showed that straw decomposition rate decreased with time under all treatments, but fast in the early stage and slow in the late stage. Moreover, AWCD, especially that related to labile carbon fractions, such as carbohydrates, decreased significantly. The amount of cyanobacteria addition affected straw decomposition rate, AWCD and αdiversity. Particularly, the highest straw decomposition rate was found under HBA within 30 days of decomposition, whereas which was occurred under LBA after 30 days of decomposition, meanwhile, the lowest straw decomposition rate appeared under MBA and HBA. The random forest analysis suggested that, during the 90 days of decomposition, the microbial degradation of recalcitrant C source, such as polymers, was the dominant factor responsible for the acceleration of straw decomposition under low amount of cyanobacteria addition, which may relate to the “microbial nitrogen mining” mechanism under low nitrogen condition. This study provides a fundamental understanding and a technical support on straw decomposition in paddy soils under cold-temperate climate.