A mercury ion sensitive fluorescent functional monomer was synthesized based on rhodamine 6G, and two highly-effective approaches about the research and development of novel macroscopic hydrogel sensor were reported. The monomer was utilized to synthesize hydrogel sensors by free radical polymerization and guest–host interaction. Hydrogel sensors have prominent selectivity to Hg²⁺ and can be tailored and reused, which are capable of detecting Hg²⁺ sensitively in flowing and standing water environment with satisfactory performance. This work is expected to open an avenue to construct novel fluorescent analysis method for Hg²⁺ detection.