Poly adenine (Poly A)-templated gold nanoclusters (Poly A-AuNCs) have many advantages, such as facile and fastly synthesis, excellent fluorescence intensity and good photostability. Based on Poly A-AuNCs, a sensitive, simple, rapid and novel fluorescent strategy was developed for the detection of Hg²⁺. AuNCs were prepared by heating-assisted methods with sodium citrate as a reductant. The AuNCs have been revealed by fluorescence spectra and transmission electron microscopy (TEM). TEM showed that the AuNCs were spherical in shape and well dispersed. The average size of AuNCs is approximately 7 nm. The fluorescence spectra showed that AuNCs emitted strong blue fluorescence with maxima fluorescence emission wavelengths at 471 nm upon excitation at 280 nm. Additionally, the AuNCs were highly stable and little fluorescence change was observed after storing at 4 ℃ for 1 month. In the presence of Hg²⁺, the fluorescence of AuNCs was quenched effectively due to the high affinity between Hg²⁺ and gold. The pH and reaction time was investigated. The fluorescence of AuNCs were insensitive to pH. The quenching reaction between Hg²⁺ and AuNCs was very fast and was completed within the first 1 min. Thus, the fluorescence was recorded after the simple mixture of Hg²⁺ and AuNCs. Under optimum conditions, a series concentration of Hg²⁺ was detected. The linear equation is y=-335.57x+541.35. The relative fluorescence intensity displayed a good linear relationship with Hg²⁺ concentration in the range from 0.01 to 1 μmol·L^-1 with the detection limit of 3 nmol·L^-1. Nine metal ions were added in the system to assess the sensing selectivity. Furthermore, the assay was successfully applied for the detection of Hg²⁺ in environmental water samples with good recoveries from 95.33%~103.8%, and the relative standard deviation was lower than 4%. Thus, this novel strategy was rapid, simple and have high sensitivity and good selectivity for Hg²⁺.