<p>Developing new porous polymer to separate uranium in water is an urgent task. In our work, a novel hypercross-linked porous polymer bearing phosphonate ligand (HCP-PL) was constructed by a simple Friedel–Crafts alkylation strategy. The obtained polymer HCP-PL was showed an excellent uranium adsorption capacity and outstanding selectivity in aqueous solution. The adsorption isotherm model is consistent with Langmuir monolayer isotherm. It is worth mentioned that the maximum uranium adsorption capacity (<i>q</i>_<i>max</i>) by HCP-PL reached up to 553.4&#xa0;mg/g, which was much higher than most reported porous materials. The polymer HCP-PL exhibits a pronounced selective adsorption capacity for uranium in the existence of other impurity ions, including Cu²⁺, Co²⁺, Sn²⁺, La³⁺, Ce³⁺, and Pb²⁺. This excellent selectivity is attributed to the strong affinity between uranium and phosphonate ligand, as demonstrated by experimental results and by density functional theory calculations. Moreover, the porous polymer HCP-PL remained removal uranium percentage of 82% after 5 adsorption–desorption cycles. Therefore, the novel hypercross-linked porous materials bearing phosphonate ligand have good application prospects in the removal of uranium ions in water.</p>