In this work, magnetic Nb-substituted crystalline silicotitanate (mag-Nb-CST), which can be used for separation of Sr2+ and Cs+ from aqueous solution, is successfully synthesized by embedding amine-functionalized Fe3O4 into the Nb-substituted crystalline silicotitanate (Nb-CST), being characterized by various techniques such as XRD, SEM, EDS, XPS, and VSM. The studies on the adsorption behaviors show that the adsorption process reaches equilibrium within about 8 h, and the maximum adsorption capacity on mag-Nb-CST is 14.38 mg g(-1) at pH 11.00 for Sr2+, and 11.18 mg g(-1) at pH 4.00 for Cs+, respectively. Besides the excellent selectivity towards Sr2+ and Cs+ over various lanthanides and actinides, the pH dependence on the adsorption capacity suggests a possibility to separate Sr2+ and Cs+ from each other by simply adjusting pH. Mag-Nb-CST is able to remove most of the Sr2+ and Cs+ at ppb level. Even in real seawater, it is able to remove 94.19% of Cs+. Moreover, mag-Nb-CST shows good acid-resistance and radiation stability. The crystal structure and morphology remained almost the same after gamma-irradiation with an adsorbed dose up to 435.8 kGy. Most importantly, only less than 0.1% of Fe is leached out from mag-Nb-CST at pH > 2, indicating that the embedded Fe3O4 nanoparticles are protected from corrosion by the coated Nb-CST, which is in favor of elimination of Sr2+ and Cs+ in acidic solutions. Thus, mag-Nb-CST is a promising adsorbent in the treatment of acidic radioactive liquid wastes, and the purification of contaminated seawater.