Abstract: Due to the lack of regulations on the resources of biomass raw materials, some crop straw grown in contaminated farmlands may be used to produce biochar with high content of heavy metals. Therefore, it is of great significance to study the chemical forms and release characteristics of endogenous heavy metals in biochar and to reduce the environmental risks due to the unreasonable biochar applications. In the study, the rice straws collected from contaminated (Jiuniu Village near the Guixi Copper Smelter in Guixi City, Jiangxi Province)and clean soils (Yingtan Red Soil Ecological Experimental Station, Chinese Academy of Sciences) were used to produce biochars named with JN-biochar and HR-biochar, respectively. The chemical fractions of Cu and Cd in biochar were analyzed by BCR sequential extraction, and the effects of solid liquid ratio and pH on the release of Cu and Cd were investigated as well. The research results showed that the concentrations of Cu and Cd in JN-biochar were 119.99 and 3.83 mg/kg, respectively, which were significantly higher than those of HR-biochar (19.50 and 0.96 mg/kg). Although the acid soluble content of copper and cadmium in JN-biochar was higher than HR-biochar, the ratios of stable state (oxidable and residual forms) to total Cu and Cd in JN-biochar (Cu 80.3%, Cd 76.7%) were higher than those of HR-biochar (Cu 53.2%, Cd 48.0%). The high solid-liquid ratio and lower solution pH increased the leaching toxicity of Cu and Cd in the biochar. Especially for JN-biochar at solid-to-liquid ratios of 1:20 and 1:60, the concentrations of Cu and Cd in leachate are higher than the limit of the Grade Ⅱ of Quality Standard for Groundwater (GB/T 14848-2017). The dynamics and cumulative-release experiments showed that Cu and Cd in both biochars can be released rapidly in a short time and then gradually increased, and the release of Cu and Cd in JN-biochar was higher than that of HR-biochar. This study suggested that the availability and environmental risk of Cu and Cd in rice straw biochar from the contaminated soil were higher than those from clean soil.