High ionic conductivity is the key point in the development of new solid‐state electrolytes. Herein, a combining strategy of anion (O²⁻) doping and structure distortion is applied to enhance the Li⁺ ion conductivity in Li₁₅P₄S₁₆Cl₃, thus converting the nonionic conductor into fast ionic conductor. Solid‐state ⁶Li nuclear magnetic resonance analysis shows redistribution of Li⁺ ions in Li₁₅P₄S₁₆Cl₃ with O²⁻ doping or local structure distortion via ball milling, indicating energy changes at different lithium sites. As a result, the activation energy is reduced from 0.50 to 0.35 eV for the ball‐milled Li₁₅P₄S_15.6O_0.4Cl₃, and the ionic conductivity is enhanced from 10⁻⁹ to 10⁻⁴ S cm⁻¹. The electrochemical stability of Li₁₅P₄S_15.6O_0.4Cl₃ is broadened at the anode side as well. The symmetric cell Li|Li₁₅P₄S_15.6O_0.4Cl₃|Li can cycle more than 1000 h with negligible voltage increase. The LiCoO₂|Li₁₅P₄S_15.6O_0.4Cl₃|Li‐Si all‐solid‐state battery demonstrates an initial capacity of 106 mA h g⁻¹ and retains 92% capacity after 200 cycles at 0.5 C, highlighting excellent rate performance and electrochemical stability.