Cancer remains a formidable global health challenge, necessitating precise diagnostic tools. The KRAS gene, particularly the G12D mutation, holds pivotal clinical significance across various cancer subtypes. Existing DNA sensing techniques using single-stranded and double-stranded oligonucleotide probes are susceptible for enzymatic degradation, especially in complex biological samples like blood or serum. To address these challenges, we present an innovative approach that involves tuning assembled DNA nanoarchitecture into a disassembled state to accurate and ultrasensitive detection of the KRAS G12D mutation. Our method utilized a pair of multifunctional assembly hairpin probes to construct a DNA wire with both of a fluorophore labeled probe and a quencher labeled probe attached. Upon binding with the target DNA, the DNA wire undergoes gradual disassembly, separating all fluorophores from quenchers to significantly amplify fluorescence signals. This approach offers exceptional sensitivity and reaction kinetics, capable of detecting the KRAS G12D at femtomolar levels, and demonstrates outstanding specificity in discriminating single-base mutations. Furthermore, the DNA nanoarchitecture with well-demonstrated ability to nuclease resistance makes our method exhibits robustness in detecting KRAS G12D mutation in human serum samples. This innovative approach represents an advancement in cancer diagnostics, providing a promising tool for early disease diagnosis and biomedical study.