The broken symmetry of Janus nanostructures (JNs) provides a distinctive means to express drastically different chemical and physical characters within a single particle and acquire emergent properties usually inconceivable for homogeneous or symmetric nanostructures. In spite of their tremendous application potential, considerable challenges are encountered in identifying pathways to synthesize or assemble JNs with a controllable geometry and morphology. Here, we exploit the reverse process of growth, i.e. silver etching, to quantitatively control the structural and optical properties of the DNA-mediated Au-Ag JNs. The transmission electron microscopy and optical measurements, along with numerical simulations, present a comprehensive view of the etching dynamics and a detailed analysis of the influencing factors that provide handles for regulating the silver etching rate and progress. In addition, a novel type of composite JN is proposed and a model system is designed and engineered through dynamical control of the etching and DNA-hybridization processes.