β-Li3PS4 is a solid electrolyte with high Li+ conductivity, applicable to sulfide-based all-solid-state batteries. While a β-Li3PS4-synthesized by solid-state reaction forms only in a narrow 300-450 °C temperature range upon heating, β-Li3PS4 is readily available by liquid-phase synthesis through low-temperature thermal decomposition of complexes composed of PS43- and various organic solvents. However, the conversion mechanism of β-Li3PS4 from these complexes is not yet understood. Herein, we proposed the synthesis mechanism of β-Li3PS4 from Li3PS4·acetonitrile (Li3PS4·ACN) and Li3PS4·1,2-dimethoxyethane (Li3PS4·DME), whose structural similarity with β-Li3PS4 would reduce the nucleation barrier for the formation of β-Li3PS4. Synchrotron X-ray diffraction clarified that both complexes possess similar layered structures consisting of alternating Li2PS4- and Li+-ACN/DME layers. ACN/DME was removed from these complexes upon heating, and rotation of the PS4 tetrahedra induced a uniaxial compression to form the β-Li3PS4 framework.