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White pollution caused by the massive discharges of waste expanded polystyrene (WEPS) has become a critical global issue, so the effective conversion of WEPS is of great significance to reduce white pollution. Herein, adopting WEPS as raw material, three high-value-added hyper-crosslinked polymers (WEPS-HCPs) with high surface areas and abundant micro/mesoporous structures were synthesized via Friedel-Crafts alkylation reaction using dimethoxymethane, dichloroxylene and 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCB) as cross-linker, respectively. The WEPS-HCPs were used as sorbent to extract aflatoxins (AFs) and all the HCPs exhibited high adsorption capacity. For the fast separation and recovery of the sorbent, WEPS-BCB was chosen as a model HCP to be magnetically functionalized to obtain a magnetic HCP (mWEPS-BCB) for the first time. Based on magnetic solid phase extraction with the mWEPS-BCB as sorbent, a fast and sensitive method was developed to determine trace level of AFs in grain food. Under the optimal conditions, the proposed method presented wide linear range (0.008–10.0 ng g−1), low detection limit (0.002–0.015 ng g−1) and satisfactory precision (relative standard deviation < 7%). Converting WEPS into a high-value-added sorbent not only provided a facile strategy for effective recycling of WEPS, but also developed a sensitive method for accurate determination of AFs in grain food, achieving significant economic and environmental benefits. [ABSTRACT FROM AUTHOR]