In this work, we unveiled a novel magnetic biosorbent, poly (Benzyldimethyl[2-[(2-methyl-1-oxoally) oxy] ethyl] ammonium chloride)-immobilized magnetic sodium alginate composite-20 (FSSEB-20), and studied its adsorption behavior for two groups of toxic anionic organic pollutants, DS and X-3B from simulation wastewater. Results demonstrated that the Langmuir model could well describe the adsorption behavior of DS and X-3B on FSSEB-20, the maximum adsorption capacity of FSSEB-20 towards DS and X-3B are 340.58 mg g−1 and 370.85 mg g−1, respectively; according to kinetic results, the adsorption processes of both DS and X-3B on FSSEB-20 followed the pseudo-second-order kinetic model. To test the reusability of FSSEB-20, five runs of regeneration tests were performed. The removal rates of FSSEB-20 towards DS and X-3B are more than 74%. In addition to these, the surface interactions between FSSEB-20 and DS/X-3B were systematically analyzed based on the characterization results of FT-IR and XPS, which provided significant insights into the interactions between biosorbents and pollutants in wastewater. The mechanism of DS and X-3B adsorption by FSSEB-20 includes electrostatic interaction, hydrogen bonding and π-π interaction. Furthermore, this study also provides a new enlightenment for expanding the treatment of pollutants in wastewater with sodium alginate-based magnetic biosorbent.