Abstract: Metal cations as well as water are important factors to control the synthesis of MnO₂ crystal nanostructures. In this work, systemic Density functional theory calculations aboutα,β,δ-MnO2 are presented to show the importance of metal cations and water for the structure stability and energy stability of MnO₂. It is shown that the α-MnO₂ crystal and its (110) surface will crash without the tunnel cations such as K⁺, and the distance between the layers of the δ-MnO2 will be significantly lower than that of the experimental results without the interlayer metal cations and water. At the same time, α-MnO2 and δ-MnO2 can be more stable than β-MnO2 with metal cations and water, and vice versa.