A new effective method is proposed for synthesis of the Li1.5Al0.5Ge1.5(PO4)3 powdered solid electrolyte of NASICON structure with high lithium ionic conductivity. The advantage of the method consists in use of a liquid-phase precursor based on the water-soluble Ge(IV) oxalate complex. Chemical interaction in a multicomponent solution containing a liquid-phase precursor results in a target product without the formation of intermediate compounds. This makes it possible to diminish considerably the synthesis temperature (to 650°C) and duration of preparation of Li1.5Al0.5Ge1.5(PO4)3 powders owing to a better homogenization of the reaction mixture and also to simplify the technological operations. The synthesized Li1.5Al0.5Ge1.5(PO4)3 powders are studied by the XRD, DSC/TG, and IR spectroscopy methods, as well as by chemical analysis, SEM, and impedance spectroscopy. The conditions are determined for production of polycrystalline Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte with the maximum bulk ionic conductivity of 1 × 10–3 S/cm at room temperature.