To explore the impact mechanisms of direct straw returning and carbonization returning on soil carbon sequestration, nutrient availability and structural stability in paddy soils, this study conducted a long-term field experiment with four treatments, including control (CK), direct straw returning, low amount of straw carbonization, and high amount of straw carbonization. The impact mechanism of straw carbonization on the stability of soil organic carbon in paddy field was systematically analyzed. The results showed that both direct straw returning and carbonization returning could effectively increase soil pH, organic carbon, available phosphorus, available potassium, alkaline hydrolysis nitrogen and microbial biomass carbon. The organic carbon of the three treatments increased by 1.9%-12.8%, and the treatment of high amount of straw carbonization was the most significant. The increase of microbial biomass carbon was between 18.0%-27.3%, and the treatment of direct straw returning showed the most significant. The proportions of low active organic carbon and inert organic carbon increased by 2.02%-7.46% after straw carbonization. In particular, compared with CK, the proportion of highly active organic carbon decreased by 5.8 percentage points, while the proportion of inert organic carbon increased by 4.3 percentage points. After three treatments, the content of >0.25 mm water-stable aggregates and >2 mm water-stable aggregates in soil respectively increased by 6.21%-9.25% and 14.53%-18.32%. In conclusion, straw carbonization returning could significantly improve soil acidification, increase the contents of soil organic carbon and available nutrient, increase the proportion of water-stable aggregates, and enhance soil carbon sequestration capacity. Among all treatments, the effect of high amount of straw carbonization returning was the best.