Unstable electrode/solid‐state electrolyte interfaces and internal lithium dendrite penetration hamper the applications of solid‐state lithium‐metal batteries (SSLMBs), and the underlying mechanisms are not well understood. Herein, in situ optical microscopy provides insights into the lithium plating/stripping processes in a gel polymer electrolyte and reveals its dynamic evolution. Spherical lithium deposits evolve into moss‐like and branch‐shaped lithium dendrites with increasing current densities. Remarkably, the on‐site‐formed solid electrolyte interphase (SEI) shell on the lithium dendrite is distinctly captured after lithium stripping. Inducing an on‐site‐formed SEI shell with an enhanced modulus to wrap the lithium precipitation densely and uniformly can regulate dendrite‐free behaviors. An in‐depth understanding of lithium dendrite evolution and its functional SEI shell will aid in the optimization of SSLMBs. In situ optical microscopy combined with atomic force microscopy studies show that the on‐site‐formed solid electrolyte interphase (SEI) shells on deposited lithium can be distinctly captured in gel polymer electrolyte based, quasi‐solid‐state lithium‐metal batteries (SSLMBs). Direct visualization of interfacial evolution of the Li dendrites and their SEI shells provides deep insights into the surface degradation and optimization in SSLMBs.