In spite of the considerable advancements achieved in enhancing the power conversion efficiency (PCE) of lead-based all-inorganic perovskite solar cells, there persists a need for materials that are both more stable and environmentally friendly. This investigation systematically explores the structural and thermodynamic stability, and electronic properties of Ge-based all-inorganic perovskite CsGeX3 (X = Cl, Br, I) in two space groups, Pm3̄m and R3m, utilizing first-principles calculations. Introducing the novel concept of the "imaginary frequency coefficient" alongside the tolerance factor and stabilizing the chemical potential window, we collectively characterize the stability of CsGeX3 based on the phonon spectrum and phonon density of states calculations. The findings reveal that the stable phase of the Ge-based perovskite differs from that of lead-based systems, with the R3m structure of CsGeX3 being the most stable in the rhombohedral phase. Moreover, the stability of R3m-CsGeX3 can be manipulated by adjusting the halide composition with a gradual increase in stability observed as halogen atoms shift from I to Cl. This comprehensive approach, integrating the phonon spectrum, innovative measurement indicators, and tolerance factor, presents an effective strategy for designing materials that are both non-toxic and stable.