Effective separation of cesium (Cs) is of great urgency and challenge for radioactive wastewater treatment and is significant for the sustainable development of nuclear power. In this work, a novel cation-modified heteropolyacid adsorbent was designed based on an efficient and simple strategy for separating Cs from radioactive wastewater. Various characterization techniques containing SEM, EDS, XRD, FT-IR, and XPS were utilized for the microscopic characterization of the adsorbent, and multi-factors affecting the Cs adsorption were investigated, containing initial ionic concentration, contact time and pH values. The adsorbent achieved an excellent Cs adsorption amount at pH > 2. The experimental results confirmed that an ion exchange reaction occurred during the adsorption process of Cs. The adsorption of Cs reached the equilibrium state within 30 min and followed the pseudo-second-order model, indicating chemical adsorption. Overall, this study demonstrates that the cation-modified material has excellent Cs adsorption properties and provides new insights into the rational design and development of inorganic materials with exceptional performance and stability.