Mercury ions, even an ultra-trace amount in water, present a serious environmental concern. Hence, searching for cost-effective and high-performance Hg²⁺ adsorbents has acquired increasingly attention but still remained challenging. In this work, aminophosphonic acid was immobilized onto polyacrylonitrile fiber by chemical grafting approaches. The functionalized fiber (PAN_APF) possessed high adsorption selectivity and efficiency for Hg²⁺ when compared with other coexisting ions viz. Pb²⁺, Cd²⁺, Ag⁺, Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Ca²⁺ and Mg²⁺. The adsorption results revealed that PAN_APF exhibited high removal capacities for Hg²⁺ over a wide pH range from 3 to 11. The adsorption process was better described by the pseudo second-order kinetic model, indicating the chemical interaction between Hg²⁺ and active groups on the PAN_APF. Moreover, the maximum adsorption capacity as calculated from the Langmuir adsorption model of 358 mg g⁻¹ was higher than that of many other adsorbents. The PAN_APF could be reused more than 10 times and it is able to decrease Hg²⁺ below 50 μg L⁻¹ which is the maximum discharge standard for mercury containing wastewater in China. A continuous-flow process was also implemented to remove Hg²⁺. The results suggested the environmentally friendly PAN_APF could be a promising candidate for Hg²⁺ removal in wastewater treatment.