Abstract: A deep purification method for removing trace thallium in water by using polymer-based nanosized manganese oxide is disclosed. This method comprising the following steps: adjusting the pH value of the water polluted with trace thallium to 5-8.5 and filtering it, then channeling the water so treated through a packed tower of filtering bed packed with polymer-based nanosized manganese oxide so that the thallium in water can be selectively adsorbed upon the nanocomposite material, stopping the adsorption process when the thallium in water reaches the leak point, and then using mixed solution of HCl—Ca(NO3)2 or NaOH—NaClO as the desorption agent and starting desorption and regeneration process for the packing material of polymer-based nanosized manganese oxide. This effectively reduces the concentration of thallium in water from 0.01-0.5 mg/L to lower than 0.1 ?g/L, despite the much higher concentration of coexisting competitive cations such as Ca2+, Mg2+, Na+ and Si(IV).