Garnet Li6.4Ga0.2La3Zr2O12 (LGLZO) solid-state electrolyte is widely considered as a promising candidate for solid-state lithium batteries. However, the production of LGLZO powder material under real circumstances faces the challenges of production instability, safety hazards and huge energy consumption concerns. Here in this work, a novel method to synthesize cubic LGLZO powder material by in-situ constructing LiOH∙H2O on GaLaZr precursor is developed. By virtue of spray drying, a continuous nano LiOH∙H2O layer with low crystallinity is successfully coated on the surface of the GaLaZr precursor particles. Revealed by 2D Raman mapping and TEM, the intimate contact and uniform mixing have been realized between LiOH∙H2O and GaLaZr precursor. Synthesis temperature of cubic LGLZO without clear secondary phases is lowered as 850 °C. The total ionic conductivity at 303 K of LGLZO is determined to be 1 × 10−3 S cm−1 with an activation energy of 0.24 eV while the electronic conductivity is characterized to 1.9 × 10−8 S cm−1. This work enriches the synthesis method of LGLZO powder material and is expected to facilitate its safe and stable scaling-up production for solid-state lithium batteries.