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The interaction between lysozyme and cinnamic acid was investigated systematically by ultraviolet–vis absorbance, circular dichroism, fluorescence, synchronous fluorescence, and three-dimensional fluorescence spectra techniques at pH 7.40. The binding constants, quenching mechanism, and the number of binding sites were determined by the quenching of lysozyme fluorescence in presence of cinnamic acid. The results showed that the fluorescence quenching of lysozyme by cinnamic acid was a result of the formation of cinnamic acid–lysozyme complex. The hydrophobic and electrostatic interactions played major roles in stabilizing the complex; the distance r between donor and acceptor was obtained to be 2.07 nm according to Forster’s theory; the effect of cinnamic acid on the conformation of lysozyme was analyzed using synchronous fluorescence, circular dichroism and three-dimensional fluorescence spectra.