In general, the presence of shared edges of polyhedra for high-valence low-coordinated small cations is rarely seen except under extreme conditions such as high pressure. However, the ambient-pressure synthesis of KZnB3O6 built of edge-sharing BO4 tetrahedra is contrary to this. By investigating the molecular dynamics, lattice dynamics, and electronic properties via density functional theory, we studied the origin of the phase stability of the edge-sharing (es) and "corner-sharing (cs)" KZnB3O6. Lattice dynamics results show that there are no phonon anomalies that could lead to the instability of es-KZnB3O6, which is consistent with molecular dynamics analysis. For "cs-KZnB3O6", a soft mode at the G point in the phonon dispersion is identified that reflects the dynamic instability with respect to small distortions. Eigenvector analysis of the soft mode of "cs-KZnB3O6" indicates that the instability comes from the linkage of ZnO5 polyhedra rather than BOx polyhedra. Electronic property calculation indicates that the edge-sharing BO4 polyhedra connected by the longest B-O sigma bonds provide a solid framework for es-KZnB3O6. In the case of "cs-KZnB3O6", the overlong Zn-O bond possesses the smallest covalent nature and the least orbital overlap among the bonds in a ZnO5 polyhedron, and these two features of the electronic structure reduce the stability of "cs-KZnB3O6" compared to es-KZnB3O6. The electronic property calculation further confirms the results obtained from lattice dynamics analysis.