A branched DNA amplification strategy was employed to design a colorimetric aptameric biosensor using unmodified gold nanoparticles (AuNPs). First, a programmed DNA dendritic nanostructure was formed using two double-stranded substrate DNAs and two single-stranded auxiliary DNAs as assembly components via a target-assisted cascade amplification reaction, and it was then captured by DNA sensing probe-stabilized AuNPs. The release of sensing probes from AuNPs led to the formation of unstable AuNPs, promoting salt-induced aggregation. By integrating the signal amplification capacity of the branched DNA cascade reaction and unmodified AuNPs as a sensing indicator, this amplified colorimetric sensing strategy allows protein detection with high sensitivity (at the femtomole level) and selectivity. The limit of detection of this approach for VEGF was lower than those of other aptamer-based detection methods. Moreover, this assay provides modification-free and enzyme-free protein detection without sophisticated instrumentation and might be generally applicable to the detection of other protein targets in the future.