A determination method of proteins with the limit of determination at nanogram levels is proposed by using a common spectrofluorometer to detect intensity of resonance light-scattering (RLS). Proteins, including bovine serum albumin (BSA), human serum albumin (HSA), pepsin (Pep), α-chymotrypsin (Chy), lysozyme (Lys) and cellulase (Cel), can combine with quercetin (QT) in the pH range 4.10∼4.35, and produce an enhanced RLS signal at 400.0 and 470.0 nm. Optimization conditions for the protein-QT interaction were tested. Depending on the concentration of QT, different proteins in the range 0∼4.5 μg ml−can be determined with the limit of determination at nanogram levels.