Sodium superionic conductors are key components for solid-state sodium batteries which are regarded as promising energy storage devices for large-scale application due to their safety, possibility of increasing energy density, and design flexibility. Recently, a new crystalline sodium superionic conductor Na₁₁Sn₂PS₁₂ was reported with a high ionic conductivity over 1 mS/cm at room temperature. Herein, we present first-principles investigations on the new sodium superionic conductor. Our ab initio molecular dynamics (AIMD) simulations confirm the isotropic diffusion of Na₁₁Sn₂PS₁₂ and show that all Na sites participate in the diffusion comparably. This study also reveals that the presence of Na vacancy/interstitial does not affect the ionic conductivity, while isovalent substitutions of Sn site by Si or Ge significantly impede the diffusion by reducing free volumes of Na diffusion path as demonstrated by our systematic AIMD simulations. Thermodynamic calculations based on phase diagram analyses indicate that Na₁₁Sn₂PS₁₂ phase is stable even at zero kelvin but possesses relatively narrow electrochemical window of 1.16–1.92 V. It is predicted that Na₁₁Sn₂PS₁₂ forms a compatible interface with TiS₂ but reacts with Na metal to produce undesirable byproducts. We believe that our theoretical study provides a broader understanding on Na₁₁Sn₂PS₁₂ and helps guide the optimal design for application in solid-state batteries.