The development of a practical Hg­(II) adsorbent is highly important for both environmental protection and public health. Herein, an adaptive metal–organic framework (MOF; FJI-H30) has been prepared from a highly flexible ligand [tris­(pyridin-4-ylmethyl)­amine] and Co­(SCN)2 with cheap Hg­(II) adsorption sites (SCN– groups) that not only has excellent chemical stability but also can capture Hg­(II) from aqueous solution with high adsorption capacity (705 mg g–1). Moreover, it also has good anti-interference ability and can be used repeatedly and large-scale prepared. Further researches demonstrate that the relatively high Hg­(II) adsorption capacity originates from the adsorbed Hg­(II)-induced deformation of FJI-H30, and such an adaptive deformation will reduce the potential repulsive forces between the adsorbed Hg­(II) ions, enabling almost all Hg­(II) absorption sites to adsorb Hg­(II) ions. Finally, how to induce the deformation of FJI-H30 by adsorbed Hg­(II) also has been studied in detail. Our work not only provides a practical Hg­(II) adsorbent for wastewater treatment but also offers a novel strategy for the design of novel MOFs for efficient heavy-metal-ion removal.