By using UV–vis absorption spectroscopy and transmission electron microscopy, the interaction between silver TNPs and iodide was studied in this paper. We found that the plates of silver TNPs could be fused to nanoparticles in the presence of iodide, and the color changes of silver TNPs solution from blue to yellow was used to establish a novel colorimetric method for iodide detection. The principle of this colorimetric assay is not to follow an ordinary colorimetry, which is generally based on the comparison of gradual color changes with a colorimeter, but a new strategy by finding the critical color in the color change process. By using this new colormetric strategy, the accuracy and the detection range can be tuned by adjusting the gradient of silver TNPs concentration used for iodide detection. The color of prepared silver TNPs is not a critical factor for colorimetric detection. Repeatable results can easily be obtained by different batches of silver TNPs, even their colors are slightly different. Furthermore, the method has high sensitivity of 10-fold than recently reported colorimetric method of iodide detection [62] and the sensitivity can be further improved by using a spectrophotometer for detection. Therefore, we believe that the concept and principle of this colorimetric analysis is important and good for developing new colorimetric methods.