<p>All-solid-state lithium–sulfur batteries (ASSLSBs) represent a crucial frontier in energy storage research, promising higher energy densities and improved safety over traditional lithium-ion systems. Despite their advantages, ASSLSBs face significant challenges, particularly in addressing interfacial instability and mechanical issues arising from the insulating nature and volume expansion of sulfur cathodes. Since the interfaces of all-solid-state batteries cannot accommodate the large volume changes, interfacial contact issues become increasingly pronounced in systems utilizing S₈ conversion chemistry. Therefore, applying stack pressure during cell operation is considered a critical factor for optimizing the performance and cycle life of ASSLSB systems. In this study, we systematically investigated the impact of stack pressure on the electrochemical behavior of ASSLSBs under four different stack pressures. Electrochemical cycling results showed a marked difference in capacity retention—74% retention after 100 cycles at high pressure, while only 6% capacity retention was observed at low pressure. This pressure-dependent cycling performance was analyzed from two perspectives: the Li⁺/e⁻ transport properties and cathode interfacial resistance. Detailed electrochemical characterizations revealed that low stack pressure leads to contact loss and deactivation of active material, which disrupts the effective ion transport pathways and increases interfacial resistance, significantly impairing the overall performance of the cell. This work highlights the critical role of stack pressure in enhancing the electrochemical performance of ASSLSBs, providing key insights for optimizing interfacial stability and transport properties in the field of all-solid-state batteries.</p>