Sulfide-based lithium superionic conductors often show higher Li-ion conductivity than other types of electrolyte materials. This work unveils a unique Li-ion conductive behavior in these materials through the perspective of anharmonic coupling assisted Li-ion diffusion. Li hopping events can happen simultaneously with various types of lattice dynamics, while only a statistically important synchronization of motions may indicate coupling. This method enables a direct evaluation of the coupling strength between these motions, which more fundamentally decides if a specific type of lattice motion is really anharmonically coupled to the Li hopping event and whether the coupling can facilitate the Li diffusion. By a new ab initio computational approach, this work unveils a unique phenomenon in prototype sulfide electrolytes in comparison with typical halide ones, that Li-ion conduction can be boosted by the anharmonic coupling of low-frequency Li phonon modes with high-frequency anion stretching or flexing phonon modes, rather than the low-frequency rotational modes. The coupling pushes Li ions toward the diffusion channels for reduced diffusion barriers. The result from the lower temperature range (≈0-300 K) of simulation can also be more relevant to the application of solid-state batteries.