In this work, we provided a logical idea to design, synthesis and application of a high-performance electrochemical biosensor based on carboxylic multi-walled carbon nanotubes/tetrahexahedral platinum nanocrystals nanocomposites, which can realize high-performance simultaneous determination for guanine and adenine in DNA. The morphology evolution of platinum nanoparticles from nanospheres to tetrahexahedral nanocrystals was also briefly studied in our work. The carboxylic multi-walled carbon nanotubes/tetrahexahedral platinum nanocrystals nanocomposites biosensor showed excellent synergistic effects in electrocatalytic oxidation of guanine and adenine in DNA, compared with other reported sensors, our proposed nanocomposites biosensor showed wider linear range of 0.05100 umol dm(-3) and 0.05100 umol dm(-3) with lower detection limit of 0.01 umol dm(-3) (S/N = 3) and 0.01 umol dm(-3) (S/N = 3) for guanine and adenine respectively. And it also exhibited excellent selectivity, stability and reproducibility. Moreover, the proposed biosensor was successfully applied in real DNA samples determination, which has great significance in application of the bioscience and clinical diagnosis.