The invention provides a novel three-dimensional carbon material and a preparation method thereof. The preparation method comprises the following step: under an inert atmosphere or an oxygen-free condition, treating hard wood according to the following temperature change conditions: (a) heating the hard wood from room temperature to 210 to 250 DEG C at a heating rate of 0.05 to 2 DEG C/min; (b) heating the hard wood from 210 to 250 DEG C to 760 to 850 DEG C at a heating rate of 0.1 to 3 DEG C/min; (c) heating the hard wood 760 to 850 DEG C to 1,100 to 1,450 DEG C at a heating rate of 0. 5 to 4DEG C/min, and preserving the heat for 0.5 to 0.6 h; and (d) cooling the hard wood from 1,100 to 1,450 DEG C to the room temperature at a cooling rate of 0.5 to 20 DEG C/min. The three-dimensional carbon material obtained by the method contains both microcrystalline carbon and amorphous carbon and has functions of producing negative ions, removing formaldehyde, resisting bacteria and the like soas to improve the additional value of a hard wood material; and the safe and reliable carbon nano material is provided.