Mesoporous Fe3O4-loaded silver nanocomposites (Fe3O4@Ag) were simply fabricated as bi-functional nanozymes for the catalysis-based detection and removal of Hg2+ ions. It was found that the as-prepared magnetic Fe3O4@Ag could display peroxidase-like catalysis activity that could be rationally enhanced in the presence of Hg2+ ions. To our surprise, the shell of the Ag element may decrease the catalysis of the Fe3O4 to some degree. However, the Ag particles could serve as the probes for specifically recognizing Hg2+ ions and trigger increased catalysis through the formation of Ag-Hg alloys, with a decreased signal background. A high-throughput colorimetric analytical method was thereby developed based on the Fe3O4@Ag catalysis for probing Hg2+ ions in the muscles of fish by using 96-well plates, at linear Hg2+ concentrations ranging from 0.010 to 2.5 mg kg-1. Moreover, the developed colorimetric analytical method was applied to evaluate Hg2+ levels in muscle samples of different kinds of fish. Unexpectedly, an obvious difference of Hg2+ levels in muscles of four kinds of fish was discovered, with the order of snakehead (Ophicephalus argus) > largemouth bass (Micropterus salmoides) > crucian carp (Carassius auratus) > silver carp (Hypophthalmichthys molitrix), where the carnivorous fish showed higher Hg2+ levels than the omnivorous or plant-based ones. Moreover, the as-fabricated Fe3O4@Ag adsorbents with their large specific surface area and high environmental robustness could exhibit efficient Hg2+ adsorption with capacities of up to 397.60 mg g-1. A removal efficiency of 99.40% can also be expected for Hg2+ ions from wastewater, with the magnet-aided recycling of Fe3O4@Ag adsorbents. Such an Fe3O4@Ag-based colorimetric analysis and removal strategy for Hg2+ ions should find wide applications in the fields of aquatic food safety, environmental monitoring, and clinical diagnostics of Hg-poisoning diseases.