A new colorimetric assay for K-Ras gene detection was developed based on double-hairpin molecular beacon (DHMB). Utilizing this colorimetric scheme, target DNA detection and screening of point mutations were accomplished by the naked eye. The colorimetric cascade amplification platform also holds the potential for common non-oligonucleotide target detection. The DHMB-based colorimetric strategy is a promising candidate platform for diagnostic applications.