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Poor interfacial contacts of Li mental/solid‐state electrolytes (SSEs) cause high impedance and induce lithium dendrite growth, which hinders the practical viability of solid‐state batteries (SSBs). Optimizing the surface chemistry of SSEs has been widely adopted for improving the Li/SSE interfacial contact. Their surface's microstructure, another critical factor influencing the actual performance of SSBs, however, has seldom been paid attention to; and the corresponding mechanism remains unclear. Addressing this issue, the authors herein propose a "surface microstructure optimization" strategy that can significantly enhance the Li/SSE interfacial contact, and in the meantime, quantify the correlations between SSB's performance and SSE's surface roughness regarding the interfacial resistance, current focusing, critical current density (CCD), and lithium deposition. Based on these fundamental understandings, a low‐surface‐roughness SSE is developed, which shows extremely small interfacial impedance (1.7 Ω cm2) and ultra‐long stable cycling life (4500 h at 0.2 mA cm−2). This work not only comprehensively demonstrates the fundamental relationship between the surface microstructure of SSEs and their battery performance but also presents a new insight into the modulation of surface kinetics of ceramic SSEs toward achieving dendrite‐free SSBs. [ABSTRACT FROM AUTHOR]