A “load-first, calcine-later” in-situ synthesis strategy was employed to prepare FeMg-LDH@cotton stalk precursors via the urea hydrothermal method, which were then calcined to obtain FeMg-LDO@cotton stalk-based biochar (FeMg-LDO@CSBC) composites. Single-factor experimental design and response surface methodology were employed to optimize key preparation parameters, including hydrothermal time, calcination temperature, and urea/metal ratio. A regression model for adsorption performance was developed, and the adsorption mechanism for polystyrene nanoplastics (PSNPs) was elucidated by using SEM-EDS, BET, XRD, XPS and adsorption model. The results indicated that the interaction between hydrothermal time and urea/metal ratio significantly affected the adsorption performance. The optimal preparation parameter combination for FeMg-LDO@CSBC composite materials was hydrothermal time of 3.85 h, calcination temperature of 555℃, and urea/metal ratio of 4.657. Through fitting with the pseudo-second-order kinetic model and Sips isotherm model, the maximum adsorption capacity was determined to be 17.74mg/g. The adsorption performance was primarily attributed to the abundant pore structure and highly active surface sites of the FeMg-LDO@CSBC composite, the adsorption mechanism was primarily driven by a synergistic combination of π-π EDA interactions, hydrogen bonding, surface complexation, and Mg2+ electrostatic shielding. The Mg2+ ions exposed at the edges of the layers acted as local positive charge centers, effectively overcoming electrostatic repulsion to achieve highly efficient and stable adsorption. Furthermore, the composite material exhibited excellent magnetic separation performance and environmental adaptability. The research result can provide novel adsorbent materials and technical support for the high-value utilization of agricultural waste and the control of microplastic pollution.