The isotherm adsorption behavior of bovine serum albumin on the mixed-mode cellulose-stainless steel powder composite adsorbent with benzylamine as the functional ligand (named Cell-SSP-BA) was measured using batch experiments. Four separate adsorbents with different ligand densities were studied under varying salt concentrations and pH. The mixed-mode adsorbent Cell-SSP-BA showed the salt-tolerant and pH-dependent properties for protein binding. An empirical modification of the Langmuir equation has been proposed to describe the effects of both salt concentration and pH on the isotherm behaviors of mixed-mode adsorption. The results indicated that the modified Langmuir equation could correlate the protein adsorption at different salt concentrations and pH with only one set of parameters.