In this study, garden waste, polyethyleneimine, and Fe2+/Fe3+ were used as starting materials to create polyethyleneimine and magnetically modified biochar derived from garden waste (P/Fe3O4@GWBC). The best condition for the preparation of the composite was explored in order to produce P/Fe3O4@ GWBC for batch testing on Cr (VI) removal. P/Fe3O4@GWBC had a rough surface with numerous fine particles, which was a typical composite with both micropore and mesopore. The experiment’s findings indicated that the acid environment was favorable for Cr (Ⅵ) adsorption. The adsorption process could be properly described by the pseudo-second-order kinetics model. The isotherm investigation demonstrated the monolayer adsorption of Cr (VI) by the appropriate fitting utilizing the Langmuir equation. The highest Cr (VI) uptake computed using the Langmuir equation was 52.6 mg/g, ranking fourth among 11 composites. The investigation of thermodynamics revealed that the adsorption process was spontaneous and endothermic. The main mechanisms for Cr (VI) removal include electrostatic adsorption, ion exchange, chelation, and Cr (VI) reduction.