Lithium-rich antiperovskites are promising solid-state electrolytes for all-solid-state lithium-ion batteries because of their high structural tolerance and good formability. However, the experimentally reported proton-free Li₃OCl is plagued by its inferior interfacial compatibility and harsh synthesis conditions. In contrast, Li₂OHCl is a thermodynamically favored phases and is easier to achieve than Li₃OCl. Due to the proton inside this material, it exhibits interesting lithium diffusion mechanisms. Herein, we present a systematic investigation of the ionic transport, phase stability, and electrochemical-chemical stability of Li₂OHCl using first-principles calculations. Our results indicate that Li₂OHCl is thermodynamically metastable and is an electronic insulator. The wide electrochemical stability window and high chemical stability of Li₂OHCl against various electrodes are confirmed. The charged defects are the dominant conduction mechanism for Li-transport, with a low energy barrier of ~0.50 eV. The Li-ion conductivity estimated by ab initio molecular dynamics simulations is about 1.3 × 10⁻⁴ S cm⁻¹ at room temperature. This work identifies the origin of the high interfacial stability and ionic conductivity of Li₂OHCl, which can further lead to the design of such as a cathode coating. Moreover, all computational methods for calculating the properties of Li₂OHCl are general and can guide the design of high-performance solid-state electrolytes.