Microorganisms were immobilized on straw-sludge-based humus, and this material was employed to carry out remediation experiments of water samples contaminated by phenanthrene, a typical polycyclic aromatic hydrocarbon. Straw-sludge-based humus was prepared by hydrothermal humification, and the degradation bacteria were obtained by screening phenanthrene-contaminated water samples treated by adsorption with humus. The bacteria were denoted by Yarrowia lipolytica Tzyx3. The chemical adsorption and biodegradation effects of the humus-immobilized microbial inoculum for phenanthrene in water and their correlation were studied. The relative contributions of chemical adsorption and biodegradation to the phenanthrene removal rates were accurately quantified. The results showed that on the prerequisite of 9 g/L humus or humus-immobilized microbial inoculum and a similar initial number of viable microorganisms, the removal rate with humus alone was 46.72%, and the phenanthrene degradation rate with free Yarrowia lipolytica Tzyx3 strains was 57.03%. In contrast, the removal rate of phenanthrene with the humus-immobilized microbial inoculum reached 78.55%, lower than the sum of the removal rates due to adsorption with humus and degradation with free strains yet higher than that due to adsorption with humus alone and degradation with the bacteria alone.