Nowadays, the development of cadmium ion detection methods and devices in the water is expected to be easier and simpler. The maximum concentration of cadmium ion in water that allowed for human body is 0.005âppm. In this work was developed an alternative method to detect concentration of cadmium ion in water based on optical fiber spectrometry using 532ânm laser light and dithizone reagent. Determination of cadmium ion concentration was using concentric fiber bundle and flat mirror as a reflecting component. The developed method was tested to detect cadmium in distilled water in different concentration from 0.005 to 0.04âppm. The performance of the method used in this study were evaluated for its sensitivity, linearity, operation range, and limit of detection (LOD). The results showed that this method had sensitivity for 515 mV/ppm with a linearity more than 99%, operation range start from 0.005âppm to 0.04âppm, and LOD reaches 0.005âppm. It can be concluded that this simple device and easy method using fiber optic bundle concentric was effective for detection of cadmium ion concentration in the water.