Disclosed is a biogas residue-based in situ and ectopic coupled detoxification method for hexavalent chromium site and soil remediation agent. The method is applied to chromium-containing soil by formulating biogas residue, a carbon source and a sulfate into said soil remediation agent. Special construction and maintenance methods are adopted to achieve effective reduction of hexavalent chromium in deep contaminated soil. Therefore, the biogas residue-based in situ and ectopic coupled detoxification method for hexavalent chromium-containing sites cannot only be used to remediate chromium-containing sites deep in depth at a low cost and high efficiency, but also achieve recycled utilization of waste by treating waste with one another, thereby possessing significant value in promotion and practice.