Mercury ion (Hg2+) is a well-known toxic heavy metal. It has become one of the most significant environmental pollutants in the world because of its serious physiological toxicity, persistence, easy migration, and high bioconcentration. Thus, the development of methods for monitoring Hg2+is indispensable. Herein, we have designed and synthesized a new fluorescent probe, TPH, for the detection of Hg2+in the water environment. The TPH probe could quantitatively detect Hg2+between 0 and 5 μM (LOD = 16 nM), with a linear range of 0–2.5 μM. In addition, the TPH probe was used to monitor Hg2+in water samples successfully. Thus, this probe is suitable for monitoring Hg2+in the actual water environment.