A naphthalene‐containing dicarboxylic ligand, 4‐(carboxymethoxy)‐6‐methyl‐1‐(p‐tolyl)‐2‐naphthoic acid (Hcmtna), is synthesized and used to construct a coordination compound, Co(H2O)4(cmtna)2 (Co‐cmtna), through the hydrothermal strategy. The moderate rigidity and flexibility of the organic molecule result a low coordination number of cobalt ion with the ligand and provide more possibilities for potential applications. Experimental results show that the fluorescence of Co‐cmtna can be quenched by multiple objects, including antibiotics, pesticides, and metal ions, which make it possible as a multifunctional sensor. More interestingly, Co‐cmtna has interactions with human serum albumin in the phosphate‐buffered saline solution, which are detailed studied by the site binding model formula, thermodynamic formula, and Föster nonradiative energy transfer mechanism.