NASICON-type Li_1+xAl_xTi_2-x(PO₄)₃ (LATP) has garnered significant attention as a promising solid-state electrolyte for lithium-ion batteries due to its simple preparation method, low material cost, and good stability in water and air, but lithium dendrite’s formation greatly limits the applications. To elucidate the source of lithium dendrite’s formation, in this study, a combination of first-principles calculations and molecular dynamics simulations was utilized to investigate the effect of Al content on the stability, electronic and Li⁺mobility properties of the LATP surface with three Al doping concentrations (2AlTi, 4AlTi, 6AlTi). We also consider Li_1+xAl_xTi_2-x(PO₄)₃(LTP) surface for comparison. The results indicate that the (012) surface terminated with Li atoms is the most stable facet. Further the surface energy of LATP(012) decreases from 0.68 J/m² to 0.43 J/m² with increasing Al content, suggesting Al doping can effectively improve the stability of the LATP(012) surface. Electronic structure analysis reveals that the surface of LTP(012) retains the semiconductor properties consistent with the bulk phase, whereas the LATP(012) surface exhibits metallicity, which provides an electron pathway for metallic Li formation. Consequently, the metallic character of the LATP(012) surface is one reason for its lithium dendrite growth. For the Li⁺ transport properties, two different migration modes, vacancy migration and interstitial migration, were included. When Li⁺ migrates within the outermost surface, the migration barrier via vacancy is 1.67/1.69 eV for the LTP/LATP (012) surface, while the migration barrier via interstitial is 1.16 eV for LTP(012) and decreases from 1.31 to 0.87 eV with the increase of Al content for LATP(012). Obviously, within the outermost surface, Al doping can decrease the migration barrier of Li⁺. When Al doping concentration is 6AlTi, the migration barrier is lowest (0.87 eV). Nevertheless, the lowest migration barrier (0.87 eV) for Li⁺ on the LATP surface is significantly higher than its bulk minimum value of 0.34 eV. When Li⁺ migrates from the subsurface layer to the outermost surface, the migration barrier is 2.76 eV for LTP(012) and 2.05 eV, 3.20 eV, and 3.06 eV for LATP(012) with 2AlTi, 4AlTi, and 6AlTicontents, respectively. All these migration barriers are greater than 2.00 eV, which prevents Li⁺ migration from the subsurface layer to the outermost surface for both LTP and LATP surfaces. Hence, the slow Li⁺ migration represents another important factor contributing to lithium dendrite growth on the LATP surface. Fortunately, increasing the Al doping concentration can reduce the migration barrier of Li⁺ and thus enhance its diffusion performance on the LATP surface. Molecular dynamics simulations further reveal that the diffusion behavior of Li⁺ on the LATP surface is influenced by a combination of factors, including Al content, Li⁺ occupancy, and ambient temperature. In particular, LATP(012)/6AlTi, LATP(012)/4AlTi, and LATP(012)/2AlTi possess the highest Li⁺ diffusion coefficient at 900 K, 1100 K, and 1300 K, respectively. Besides, Li⁺near the Al doping site is easier to diffuse on the LATP(012) surface. Thus, our study suggests that by varying Al content, Li⁺ occupancy positions, and the temperature, Li⁺ diffusion performance of LATP(012) can be favorably modified, and consequently inhibiting the formation of lithium dendrites on the LATP(012) surface.