Solid‐state lithium batteries (SSLBs) with high safety have emerged to meet the increasing energy density demands of electric vehicles, hybrid electric vehicles, and portable electronic devices. However, the dendrite formation, high interfacial resistance, and deleterious interfacial reactions caused by solid–solid contact between electrode and electrolyte have hindered the commercialization of SSLBs. Thus, in this review, the state‐of‐the‐art developments in the rational design of solid‐state electrolyte and their progression toward practical applications are reviewed. First, the origin of interface instability and the sluggish charge carrier transportation in solid–solid interface are presented. Second, various strategies toward stabilizing interfacial stability (reducing interfacial resistance, suppressing lithium dendrites, and side reactions) are summarized from the physical and chemical perspective, including building protective layer, constructing 3D and gradient structures, etc. Finally, the remaining challenges and future development trends of solid‐state electrolyte are prospected. This review provides a deep insight into solving the interfacial instability issues and promising solutions to enable practical high‐energy‐density lithium metal batteries.