In this work, a new type of hyper-crosslinked phosphate-based polymer (HCPP) polymerized by bis(2-methacryloxyethyl)phosphate has been developed for uranium and rare earth element (REE) extraction in an aqueous solution. The influence of the pH value, contact time, initial concentration, temperature, and competing ions on uranium adsorption of HCPP is investigated in detail. HCPP exhibits a maximum uranium adsorption capacity of up to 800 mg g(-1) at pH = 6.0 and excellent selectivity toward uranium adsorption over coexisting ions, because of the high affinity between HCPP and uranium ions and dense phosphate groups on the backbone. It also demonstrates high adsorption performance in both simulated seawater with a high salt concentration and a real nuclear industrial effluent. Besides, the crosslinked network structure of HCPP endows this polymer with high chemical stability and reusability. Furthermore, the adsorption mechanism is probed by energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared measurements. It is confirmed that the adsorption of uranium on the adsorbent originates from the interaction between phosphate groups and uranium ions. Meanwhile, HCPP also displays high REE adsorption capacities. This work indicates that the phosphate-based HCPP could be utilized as a promising adsorbent for the effective removal of uranium and REEs from aqueous solution.