The search for next-generation solid-state superionic conductors has attracted significant attention. Among Na superionic conductors, Na11Sn2PS12 has been reported to have a room temperature ionic conductivity of 1.4 mS/cm. In this study, we employ density functional theory to study the stability of Na11Sn2PS12 and further explore the substitution of Sn with Ge. Our results indicate that Na11Ge2PS12 is more stable than Na11Sn2PS12. Furthermore, substituting Sn with Ge increases the band gap, improves the room temperature ionic conductivity by a factor of 2, and lowers the activation energy of Na hopping. Statistical analysis suggests that Na11Ge2PS12 has a faster diffusion along the ab-plane compared to the c-axis. The Na diffusion in Na11Ge2PS12 appears to occur with two different mechanisms depending on temperature: 1) an ion hopping process at lower temperatures (<800 K); 2) a fluid-like distribution of Na ions at higher temperatures (>1000 K). The computations suggest that Na11Ge2PS12 is a promising candidate as a solid Na electrolyte due to its high room temperature ionic conductivity and phase stability. In light of these simulation results, we expect to stimulate further experimental studies on Na11Ge2PS12.