Along with the widening application and growing market size of energy storage devices, the development of costeffective rechargeable batteries with a high level of operational safety has become a major challenge. To this end, an all-solid-state battery (ASSB), which is composed of a thin film instead of a liquid, is an attractive candidate. In this study, we investigated a facile method for preparing sodium superionic conductor structured Na1+xZr2SixP3-xO12 (0 <= x <= 3, NZSP). Various attempts were made to improve the sinterability of NZSP, but the results are still unsatisfactory. We employed the reaction sintering method so that the phase formation and densification proceeded simultaneously, resulting in the densification of NZSP with minimal impurities. Furthermore, we successfully substituted rare-earth elements (REs) into the Zr site of the NZSP to tune its structural properties in the nanoscale and improve its ionic conductivity. Electrochemical impedance spectroscopy results confirmed the improvement of the ionic conductivity of both the pristine NZSP and the RE-doped variant, indicating the effectiveness of reaction sintering. When reaction sintering and RE substitution were employed together, La-doped NZSP was an attractive solid electrolyte for application in ASSBs. Our results highlight the effectiveness of reaction sintering for obtaining an impurity-free and highly dense multicomponent compound.