The combination of radiotherapy and chemotherapy is a common and useful treatment mode for tumours. But traditional methods inevitably lead to a variety of side effects. A drug delivery system (DDS), which has good biocompatibility and strong anti-tumour ability, is expected to solve this problem. Studies have shown that Ce-based nanoparticles (NPs) have good radiosensitization effect through the photoelectric effect. Hence, cisplatin-loaded LiLuF₄ :Ce3+scintillation NPs (NP + Cis) were first constructed in this study, which was synthesized by the crystal precipitation method and characterized by transmission electron microscopy (TEM). Subsequently, its toxicity was verified, and the radiosensitization effect and basic radiosensitization mechanism on tumour cells and tumour-bearing mice were researched. Results showed that NP + Cis triggered massive DNA damage and effectively inhibited cell viability in vitro under the exposure of X-ray irradiation (IR). Moreover, the experiments in vivo showed that the NP + Cis had higher biosafety, which could absorb enough irradiation and produce a synergistic inhibitory effect on tumours through the releasing of Cis. NP + Cis can improve the performance of DDS in chemoradiotherapy.