In situ scanning electron microscopy (SEM) offers a good way to investigate the structural evolution during lithiation and delithiation processes. In this paper, the dynamical morphological evolution of 3D-line-structured/unstructured Si/C composite electrodes was observed by in situ SEM. The investigation revealed the microstructural origin of large charge capacity for 3D-line-structured anodes. Based on this proposed mechanism, a coarse optimization of 3D-line-structured anodes was proposed. These results shed light on the unique advantages of using an in situ SEM technique when studying realistic bulk batteries and designing 3D electrode structures.