AbstractLi1.3Al0.3Ti1.7(PO4)3 (LATP) is one of the most attractive solid‐state electrolytes (SSEs) for application in all‐solid‐state lithium batteries (ASSLBs) due to its advantages of high ionic conductivity, air stability and low cost. However, the poor interfacial contact and slow Li‐ion migration have greatly limited its practical application. Herein, a composite ion‐conducting layer is designed at the Li/LATP interface, which a MoS2 film is constructed on LATP via chemical vapor deposition, followed by the introduction of a solid polymer (SP) liquid precursor to form a MoS2@SP protective layer. This protective layer not only achieves a lower Li‐ion migration energy barrier, but also adsorbs more Li‐ion, which is able to promote interfacial ion transport and improve interfacial contacts. Thanks to the improved migration and adsorption of Li‐ion, the Li symmetric cell containing LATP‐MoS2@SP exhibits a stable cycle of more than 1200 h at 0.1 mA cm−2. More remarkably, the capacity retention of the full cell assembled with LiFePO4 cathode is as high as 86.2% after 400 cycles at 1 C. This work provides a design strategy for significantly improving unstable interfaces of SSEs and realizing high‐performance ASSLBs.