Power capability of solid state lithium sulfur (Li-S) batteries is severely weakened due to high impedance between electrodes and solid electrolyte. To solve this issue, herein we come up with a room temperature operating (20 °C) solid Li-S battery technology enabled by ionic liquid (IL) as interfacial modification. With IL, a significant decrease of interfacial resistance is observed from 3540 to 39 Ω cm2 in symmetric cell, which can cycle well at a high current density of 0.5 mA cm−2 with a small over potential of 300 mV (20 °C). On this basis, solid state Li-S batteries are fabricated with IL modification, and performed an excellent electrochemical behavior at 0.2 mA cm−2 with the specific capacity of 340 mAh g−1. Distinctly, the thin IL layer presents electrochemical and physical stability during the cycling and plays a flowable and seamless interfacial layer in solid state Li-S batteries.