In order to study the chemical speciation of heavy metal cadmium(Cd)and lead(Pb)in mixed-contaminated paddy soil and the response to the amendment by biochar,the fractional extraction was used to the speciation analysis of Cd and Pb in the mixed-contaminated soils amended biochar with different application rate,and the stability mechanism was also discussed.The results show that the pH value and the content of soil organic carbon(SOM)increased by 0.18-0.64 units and 19.3%-42.8%,respectively.Biochar application could effectively promote transformation of exchangeable Cd(decrease 9.6%-13.7%)into residual fraction(increase 6.3%-8.2%),and exchangeable Pb(decrease 2.4%-3.7%),carbonate bound form Pb(decrease 4.2%-6.5%)into residual fraction(increase 7.3%-10.2%),respectively,resulting in significant reduction of heavy metals bioavailability and mobility.Simultaneously,biochar application can increase the rice yield of 15.6%-28.2%and decrease Cd and Pb content in the rice,and the biochar application rate of 20 t/hm2 was the most effective treatment for the remediation of Cd and Pb in mixed-contaminated paddy soil.The different Cd and Pb states were significant correlation with soil pH and SOM.Biochar contained large amounts of alkaline material and hydroxyl,carboxyl and organic functional groups,which could fix Cd and Pb effectively by complexation,precipitation,and other chemical mechanisms.It showed that remediation of the paddy soils by biochar could effectively reduce the bioavailability and ecotoxicity of Cd and Pb,and provided a theoretical basis for the remediation of heavy metal mixed-contaminated paddy soils.