Abstract: For clear biochar on the mechanism of the distribution of Cd forms in soil aggregates, a pot experiment was conducted out for straw biochar as raw material to study the effects of straw biochar on the enrichment of Cd in different size fractions of micro-aggregates and the transformation of Cd forms in micro-aggregates of rhizosphere and non-rhizosphere. The results indicated that when Cd was added to the soil, it was mainly distributed in the soil of different particle size micro-aggregates, and increased with the decrease of particle size. According to the order from big to small, the enrichment sequence was < 0.01, 0.05-0.01, 0.25-0.05, >2 mm. After adding biochar, the enrichment coefficient (P < 0.05) of Cd was significantly decreased in different particle size micro-aggregates. Compared with the control, the enrichment coefficients of >2, 0.25-0.05, 0.05-0.01 and < 0.01 mm decreased by 0.04-0.16, 0.04-0.15, 0.07-0.17, 0.06-0.21 times respectively. The contents of rhizosphere and non-rhizosphere Cd in different particle size micro-aggregates were mainly dominated by residual fractions, and the content of Cd in each form was mainly concentrated in small grain micro-aggregate (< 0.01 mm) under different treatments. Rhizosphere and non-rhizosphere Cd content of exchangeable, Fe Mn oxides and carbonate bound and organic bound decreased in different degrees, and residual Cd increased. Compared with the control, rate of decline of the organic bound Cd was highest in the rhizosphere and non-rhizosphere of different micro-aggregates in various forms that were 49.5% and 62.3%, respectively in < 0.01 and 0.05-0.01 mm micro-aggregates respectively under Cd1B10 treatment. The rate of reduction of the residual form Cd were the highest that were 19.8% and 20.5%, respectively in 0.25-0.05 and 0.05-0.01 mm micro-aggregates respectively under Cd1B10 treatment. However, the percentage of residue state tended to increase under each treatment, with the highest percentage reaching to 74.0% and 78.2% respectively. The results showed that Cd was mainly transformed into residual state after soil entry, and the bioavailability of Cd decreased in soil. The Cd-contaminated of low concentration (1 mg/kg) and the highest biochar application (10 g/kg), the effect was significant. Studies have shown that the application of high biochar(10 g/kg) can reduce the content of Cd in soil micro-aggregates of different sizes and contents of different forms of Cd, and the lower concentration of Cd, particle size is smaller, the more significant effect.