The detection of thiocyanate (SCN^-) is particularly important in industrial effluents and biological fluids because of the toxic nature of SCN^-. Herein, a metal-insulator-metal (MIM) resonator for visual detection of SCN^- is presented based on a poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) brush. The MIM resonator exhibits obvious color change as the concentration of SCN^- changes, which can be easily distinguished by the naked eyes. In addition, the as-prepared MIM resonator also shows the advantages of good anti-interference, excellent reusability, and fast response rate. Combining the above advantages, the proposed MIM resonator may provide a broad perspective for a wide variety of visible-light applications.