Mercury pollution has always been a huge threat to human health due to its significant toxicity. Thus, it's the continuing goal to obtain new mercury detection techniques that are cost-effective, operational stable, performance efficient, and applicable to the environmental and biological milieus. In this research, the soluble pigment pyocyanin with anti-bacterial and anti-fungal activities, the biosynthesis pathway of which was engineered under the regulation of Hg-dependent transcriptional activator MerR, was firstly used as the visual detection signal in the whole-cell biosensor. The engineered biosensor displayed optical sensing window and a good linearity for Hg in the range of 25-1000 nM, and the detection limit could reach as low as 10 nM. It permitted on-site detection of bioavailable Hg with extraordinary selectivity and could resist the interferences of extra metal ions. What's more, the developed biosensor performed function well in a wide pH range (pH 4-10) as well as the environmental water. By fully imitating and utilizing the biosystems from nature, the engineered colorimetric biosensor has great economic and performance advantages over most chemosensors as well as whole-cell biosensors in the practical application of detecting Hg in the contaminated aquatic systems.