Quantitative morphological evolution is of great importance in nanochemistry. In this work, morphology of silver nanotriangles (AgNTs) is quantitatively evolved under the guidance of DNA. First, intact AgNTs are prepared relying on the protection of horseradish peroxidase. Then different regions of AgNTs are sequentially etched by C-rich DNA, leading to DNA-guided postshaping of AgNTs. In combination with atomically resolved images and theoretical simulation, a model is established to track the postshaping process. Since real-time morphological evolution of AgNTs is determined with spectra, a series of AgNTs with specific corners can be obtained by controlling incubation time. The DNA-guided postshaping is sequence and structure dual-dependent, and a mechanism is proposed based on metal-base interaction, surface energy of faces, and freedom of DNA structure. In addition, the postshaping is further used to design DNA-mediated biosensors. This study provides a precise and quantitative method of controlling morphology of anisotropic metallic nanomaterials.