Reducing particle size of sulfide electrolytes to nanoscale is essential for preparing ultrathin sulfide electrolyte membrane of all-solid-state batteries. However, there lacks effective way to synthesize sulfide electrolyte nanoparticles with good morphology uniformity. In this work, a dual precursor sol route is developed to speed up the synthesis of lithium thiophosphate (LPS) from tens of hours by conventional liquid-phase routes to <50 min. The promoted reaction kinetics inhibit the overgrowth of the product particles so that monodispersed LPS nanoparticles with sizes of 44 +/- 6 nm are prepared. Membranes fabricated from the nanoparticles achieve a thickness of 0.5-1.0 mu m, which are reduced by one order of magnitude in comparison with those of reported sulfide electrolyte membranes. Moreover, the monodispersity makes the nanoparticles closely packed in the green membrane, so that the membrane can be self-densified without applying pressure, whereas 100-MPa-level pressure is indispensable for conventional membrane fabrication technologies. The ultrathin LPS membrane is also successfully employed in all-solid-state LiFePO4 thin film batteries. This work not only provides a heuristic strategy to synthesis sulfide electrolyte nanoparticles but also develops a pressing-free fabrication route of sulfide electrolyte membranes, which is eagerly demanded for high-energy-density all-solid-state batteries.