Nowadays, the majority of the studies on the substitution are focused on cations (such as Y³⁺, Ti⁴⁺, P⁵⁺, etc.) in Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), while there are few studies on the substitution of anion O²⁻. In this work, the modified LATP with a series of LiCl (LATPClx, x = 0.1, 0.2, 0.3, 0.4) additives is prepared to enhance ionic conductivity. The successful introduction of Cl⁻ makes the length of the c axis decrease from 20.822(2) to 20.792(1) Å, and the bulk conductivity of 2.13 × 10⁻³ S·cm⁻¹ is achieved in LATPCl0.3. Moreover, the Al/Ti-O₁/Cl₁ and Al/Ti-O₂/Cl₂ distance decrease, while the Li₁-O₂/Cl₂ distance increases. Lithium ions migrate more easily in the nanochannel of M3-M1-M3. In addition, the LiCl additive increases the relative density and the grain boundary conductivity of LATPClx compounds. Naturally, a higher ionic conductivity of 2.12 × 10^–4 S·cm⁻¹ and a low activation energy of 0.30 eV are obtained in LATPCl0.3. Correspondingly, the symmetric cell exhibits a low overpotential of ±50 mV for over 200 h in LATPCl0.3. The solid-state Li|LATPCl0.3|NCM811 (NCM811 = LiNi_0.8Co_0.1Mn_0.1O₂) battery exhibits high initial capacity 185.1 mAh·g⁻¹ with a capacity retention rate of 95.4% after 100 cycles at 0.5 C. This result suggests that LiCl additive is an effective strategy to promote electrochemical properties of LATP solid electrolyte and can be considered for reference to other inorganic solid electrolytes systems.