The interactions of hen egg lysozyme (HEL) and bovine serum albumin (BSA) with an azobenzene polymer (P(AZO-co-AA)) were investigated by fluorescence spectroscopy. The azobenzene chromophore could strongly quench the intrinsic fluorescence of the proteins through the Forster resonance energy transfer (FRET) and the quenching effect was dependent on the pH values since the polymer also contained pH-sensitive carboxylic groups. The fluorescence quenching by azobenzene shows great potential applications in protein sensing.