A novel colorimetric strategy for protein detection was developed based on unmodified gold nanoparticles (AuNPs) and terminal protection from a target protein. A hairpin-structured DNA probe with a biotin modification at the 3' terminus combined with streptavidin (SA) by the high affinity between biotin and SA and resisted degradation from exonucleases III (Exo III), which selectively digested double-stranded DNA from the 3'-terminus. The remaining hairpin DNA lost the capacity to protect AuNPs from salt-induced aggregation, resulting in a color change of AuNPs from red to blue. In the absence of SA, Exo III degraded the hairpin DNA into single-stranded DNA, which adsorbed on the surfaces of AuNPs and prevented aggregation, keeping AuNPs with a red color. The state of AuNP aggregation can be discriminated by colorimetry and also can be measured by UV-vis spectroscopy. Our proposed method provided great selectivity and high sensitivity with a detection limit of 0.24 pmol for SA detection. Moreover, the assay avoided modification, enzymatic amplification and special equipment, and also offered the advantages of being simple, cost-effective, and sensitive. This novel strategy holds great promise for the detection of other proteins by small molecule interactions with proteins.