The objective of this study was to investigate the effect of straw return on the formation of Fe-Mn colloids in arsenic-contaminated soils and its subsequent influence on arsenic behavior. It was observed that organic matter (SD) resulting from straw decomposition interacted with iron/manganese (hydr)oxides (Fe/Mn (hydr)oxides) present in the soil, leading to the formation of colloidal particles. These particles significantly influenced the fixation and release of arsenic. The experimental results indicated that an increase in SD content facilitated the formation of colloidal particles. The highest concentration of colloidal particles was observed at a C/Fe-Mn ratio of 2.2, which significantly reduced the bioavailability and mobility of arsenic in the soil. The increase in SD content also diminished the depositional attachment efficiency of SD/Fe-Mn, thereby enhancing its migration through the soil. The actual field soil-filled column experiments further demonstrated that the content of SD significantly influenced arsenic bioavailability and mobility. Specifically, at a C/Fe-Mn ratio of 2.2, the inhibition of arsenic migration and bioavailability was found to be 1.46 times more effective compared to a C/Fe-Mn ratio of 0.4. Therefore, the return of straw to the field represents an effective soil remediation strategy for mitigating the bioavailability of arsenic by modulating the C/Fe-Mn ratio. This approach offers a novel perspective on strategies for heavy metal remediation.