Natural gas hydrate accumulation and exploitation is a research focus on the development and application of new energy. However, a group of key issues involving the formation and dissociation characteristics of natural gas hydrate in marine sediments and the effect of salt ions on the hydrate stability remains to be solved. Based on the in situ sampling technique, the formation and dissociation processes of methane hydrate in montmorillonite contained saline solution was explored by in situ scanning electron microscopy and energy dispersive spectrometry. Furthermore, the morphology and ion distribution of methane hydrate during the formation and dissociation processes in montmorillonite adsorbing 0.5 mol/L NaCl solution were systematically analyzed. The results showed that the element distribution changed obviously during the hydrate formation and dissociation processes. NaCl was located on the boundary between hydrate granules caused by the salt-removing effect of hydrate formation and existed as the form of hydrated salt ions. Na+ and Cl- were distributed on the surface of montmorillonite as different layers. The surfaces of montmorillonite present the structure of independent granular while methane hydrate was formed. After the hydrate dissociation, the surfaces of montmorillonite sunk and formed some tiny gas channels. At the same time, the packing structures of montmorillonite were changed. The research demonstrated that the processes of hydrate nucleation, growth, and decomposition took place independently on the special unit between particles. Those processes had a closed relationship with the unit cell structure of hydrate.