Mg(OH)2 samples with acicular or fibrous crystallizing morphology were successfully prepared by one-step reaction method under hydrothermal conditions and indexed as a hexagonal crystal system based on the results of XRD and TEM. Moreover, based on the theoretical model of anionic coordination polyhedron growth units, the growth mechanism and growth habit of the Mg(OH)2 crystals under hydrothermal conditions were first investigated in depth. It was concluded, after calculating the stability energy of the growth unit of Mg(OH)2 crystalline, that the growth direction of the growth unit of the Mg(OH)2 crystal tends to favor a one-dimensional direction, parallel to the basic layer of the Mg(OH)2 crystal. Accordingly, we propose that natural brucite with fibred crystallizing morphology was formed under hydrothermal conditions in the earth thousands years ago.