In order to make better use of silkworm excrement (SE) resources, the soil incubation experiments were conducted in this study to investigate the effects of SE and silkworm excrement biochar (SB) on the chemical properties of paddy soil. The sequential extraction procedure (SEP) was applied to analyze the chemical forms of cadmium (Cd), and the stability and mobility of Cd in paddy soil was also evaluated by leaching test. Results showed that SE and SB could significantly (p < 0.05) increase the soil pH, cation exchange capacity (CEC) and reduce the content of the extractable Cd, especially at 10% application rate, than that in the untreated soil. The SEP elucidated that SE decreased the ratio of exchangeable fraction (F1), increased the ratio of carbonated-bound fraction (F2), Fe-Mn oxides fraction (F3) and complexed with organic matter (F4), compared with the control. However, SB reduced the ratio of F1 and F2, increased the ratio of F3, F4 and residual fraction (F5). The 10% SB treatment reduced the concentration of phytoavailable Cd by 85.5% compared to the control. The results indicated that the SB could immobilize the Cd effectively, reducing its mobilization and phytoavailability in soils.