The invention relates to a 2-benzylthio-aniline croconic acid cyanine chemical sensor capable of identifying Hg<2+> and a preparation method of the chemical sensor. The preparation method comprises steps as follows: 2-benzylthio-aniline is synthesized from 2-aminothiophenol and benzyl chloride, and a 2-benzylthio-aniline croconic acid cyanine dye compound is synthesized from 2-benzylthio-aniline and croconic acid. The chemical sensor and the preparation method have beneficial effects as follows: the synthetic process is relatively simple, reaction conditions are easy to control, and the prepared 2-benzylthio-aniline croconic acid cyanine dye has excellent optical performance and optical stability. When the 2-benzylthio-aniline croconic acid cyanine dye is used for identifying Hg<2+>, colorof a solution also changes besides change of an absorption spectrum, so that the 2-benzylthio-aniline croconic acid cyanine dye has a colorimetric identification function and facilitates detection for Hg<2+>. Besides, the 2-benzylthio-aniline croconic acid cyanine dye has good selectivity and anti-interference performance for identification of Hg<2+>.