A polarity-sensitive fluorescent probe MNP was rationally designed and synthesized with naphthalimide as the fluorophore and maleimide as the receptor for thiols. MNP is weakly fluorescent due to the photoinduced electron-transfer （PET） from the fluorophore to the receptor, and it displays evidently solvatochromic UV-vis and fluorescence spectra： the emission shifted from 495 nm in n-hexane to 545 nm in phosphate buffer solution. Michael addition reaction between thiols and the maleimide in MNP inhibited the PET process, which led to about eight-fold fluorescence enhancement. In addition, MNP showed highly sensitivity to rnercapto-containing proteins and it could detect as low as 20.4 μg/mL of BSA in PBS. MNP has potential in fluorescent imaging of thiols in living cells.