Garnets are promising solid electrolytes for developing solid state Li batteries, owing to their features of relatively high conductivity and stability against lithium metal. However, they show shortcoming of Li penetration through garnets during Li plating and stripping, which limits their practice application. Herein, we present a strategy to solve such problem by coating SnO2 films on the surfaces of the Li6 · 4La3Zr1 · 4Ta0 · 6O12 pellets. Through conversion reaction of SnO2 with Li at 200 °C, the nanocomposite layers consisting of crosslinked LixSn and Li2O are formed between the Li and the Li6 · 4La3Zr1 · 4Ta0 · 6O12 electrolytes. This leads to transition from lithiophobicity to lithiophilicity, thus greatly reducing interfacial resistance from 1100 Ω cm2 to 25 Ω cm2. Furthermore, taking advantage of suppressing volume change of LixSn alloy which is about 260%, the intermediate layers maintain integrity under the current densities of 0.2 mA cm−2 for 650 h cycles. In addition, the critical current density of Li/SnO2-Li6.4La3Zr1 · 4Ta0 · 6O12-SnO2/Li can be as high as 1.15 mA cm−2. As a proof-of-concept, this effective interface modification based on conversion reaction method contributes to a useful way of solving the Li/garnet interface problem and promoting the solid state Li batteries development.