All-solid-state batteries, renowned for their enhanced safety and high energy density, have garnered broad interest. Oxide solid electrolytes are highly anticipated for their balanced performance. However, their high Young’s modulus and inadaptability to volume change during cycling lead to poor contact and eventual battery failure. In this work, Young’s modulus of Li_1+x(OH)_xCl samples is lowered to a level comparable to that of sulfide by regulating the –OH content. As the –OH content increases, Young’s modulus of Li_1+x(OH)_xCl samples decreases significantly. This may be due to the local aggregation of –OH groups, forming cavities similar to LiOH structure, which reduces the bonding of the structure. On the premise of high Li-ion conductivity and electrochemical stability, the lowered Young’s modulus improves the contact between the solid electrolyte and the electrodes, forming a strong and stable interfacial layer, thereby improving interfacial and cycling stability. The symmetrical lithium metal cell shows excellent cycle performance of 600 h, and the assembled LiFePO₄|Li_2.4(OH)_1.4Cl|Li cell shows significantly enhanced cycling endurance with 80% capacity retention after 150 cycles. This work not only emphasizes the crucial importance of Young’s modulus in improving interface issues but also offers innovative approaches to advance the mechanical properties of solid electrolytes.