Display omitted •A new ultra-violet light induced fluorescent Ag nanocluster was used in this system.•The method can detect Br− and I− simultaneously in complicated samples.•The method for halide ions detection with high sensitivity and selectivity. A novel fluorescent Ag nanocluster (AgNCs) stabilized with carboxymethyl dextran (CMD) was prepared by the photochemical reduction of a CMD-Ag(NH3)2+ mixture under ultraviolet C (UVC) light irradiation. Interestingly, the as-prepared Ag NCs showed the maximum fluorescence near the localized surface plasmon resonance (LSPR) wavelength of AgNCs; therefore, we defined this Ag NCs as LSPR-AgNCs. This was developed as a fluorescent probe for the simultaneous determination of iodide and bromide ions (I− and Br−) in different media. The sensing mechanism is based on the unique reactions between I−/Br− and silver atom on the surface of AgNCs, which results in their fluorescence quenching. Specifically, I− was detected selectively in the presence of Br− in a 4.5×10−4M ammonia solution. At the same time, the total concentration of Br− and I− was obtained by detecting the fluorescence change in Britton–Robinson buffer (pH 6.8). This medium-selective fluorescent probe provides the highly selectively detection of I− in the range from 1.0×10−10M to 1.0×10−7M. Moreover, this sensing system could be applied to the detection of I− in kelp samples.