Paddy soil samples with different degrees of Cd/Pb contamination in south Jiangsu were collected, and an incubation experiment was conducted to study the effects of corn straw addition (fresh carbon) on the mineralization of soil organic carbon (SOC, old carbon). CO₂ fluxes, δ¹³C abundance, contents of total organic carbon and labile organic carbon fractions were measured, and the priming effect after corn straw addition was calculated. The results showed that the corn straw addition increased the mineralization rate and cumulative mineralization of SOC, and the cumulative mineralization of SOC increased by 120% for light pollution (P0), 540% for moderate pollution (P1), and 360% for heavy pollution (P2), respectively. Moreover, corn straw addition also promoted the mineralization rate of the original SOC. Compared with P0 and P1, P2 could further promote the mineralization of old carbon in paddy soil and reduce the content of dissolved organic carbon, and its relative priming effect was significantly higher than those of P0 and P1 in different incubation stages. The relative priming effects reached the highest at the 30th day, which were 47.3% for P0, 148.2% for P1, and 189.2% for P2, respectively.