Solid state batteries hold the promise of enhanced safety and higher energy density over conventional lithium-ion batteries with flammable organic electrolytes. However, advancement of solid electrolyte materials has yet to translate into practical batteries due to the need to process the powders into thin sheets with high pressure compaction and high temperature sintering. Here, a new strategy is developed for synthesizing sulfide-based solid electrolyte using low-temperature solution processing, which is a simple and potentially cost-effective way to make a thin solid electrolyte layer. By controlling the stoichiometric ratio of Li2S and S, soluble polysulfides are produced in diethylene glycol dimethyl ether, which are reacted with P2S5 to form a conductive Li3PS4 solid electrolyte. It is demonstrated that a dense solid electrolyte layer can be directly formed on Li metal with a high quality electrolyte/electrode interface, producing a solid electrolyte with promising electrochemical performance. Also, first-principles calculations are conducted to elucidate the formation mechanisms behind the soluble intermediates and the solid electrolyte layers.