For mercury (II) ion (Hg2+) fluorescent sensors, excellent water solubility is a critical property to ensure Hg2+ detection sensitivity in biological and ecological environments. Although many organic fluorescent Hg2+ sensors have been reported, only a few applied in purely aqueous media. Herein, we synthesized a simple oligo (ethylene glycol)-functionalized organic fluorescent sensor (NP-DT-OEG) for the determination of Hg2+ in aqueous media based on a specific Hg2+-triggered dithioacetal deprotection reaction mechanism. NP-DT-OEG exhibits excellent water solubility and fast response of Hg2+ with remarkable selectivity in a purely aqueous media, the fluorescence intensity increased linearly with Hg2+ concentration within the range from 0 to 20 μM and the detection limit was determined to be 6.2 nM. In practical application, NP-DT-OEG was able to determine Hg2+ in crayfish meat and serum samples. Moreover, bioimaging studies indicated that NP-DT-OEG exhibited low cytotoxicity as well as excellent cell membrane permeability, and has been successfully used for monitoring Hg2+ in living cells.