A combined remediation through washing using humic substances (HS) followed by fixation using zeolite was employed to remediate Cd-contaminated sediment. Cadmium speciation and a series of microbial endpoints were discussed to evaluate the effectiveness of remediation. The combined treatment slightly reduced soil pH and increased soil electrical conductivity and organic matter. The Cd removal efficiency ranged from 11.4 %-26.0 % by HS washing. Zeolite immobilization further increased the residual Cd by 9.0 %-16.4 %, compared with the corresponding HS washing alone. The acid-soluble Cd decreased from the original 1.266 to the lowest 0.466 mg kg(-1), while the residual Cd increased from 1.397 to a maximum of 1.595 mg kg(-1) in the combined treatments. The change of dehydrogenase was the most sensitive (the maximum increase was 220 times). The increased microbial biomass implied that sediment microenvironment was optimized. In vitro digestion results showed that Cd accessibility decreased by 3.9%-67.9%. Principal component analysis suggested that treatments related to the high-concentration HS washing, especially 2000 mg L-1 HS washing +100 g kg(-1) zeolite stabilization, had the best biological performance and it is a feasible technique to restore microenvironment of Cd-contaminated sediments. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.