The invention relates to a carbon/carbon composite material and a preparation method thereof. The preparation method comprises the following steps: dissolving or dispersing a biomass carbon source in water to prepare a carbon source water solution or suspension; adding the carbon source water solution or suspension into a reaction kettle, putting a heating tube sheathed with a carbon fiber prefabricated body into a reaction kettle, and heating to perform hydrothermal carbonization reaction; and pouring out the liquid in the reaction kettle, adding the carbon source water solution or suspension into the reaction kettle, and heating to perform hydrothermal carbonization reaction, thereby obtaining the carbon/carbon composite material. The invention provides a novel thermal-gradient hydrothermal method for preparing a carbon/carbon composite material. On the basis of the characteristics of renewable hydrothermal carbonization raw material, low synthesis temperature and environment friendliness, the method also has the characteristic of high densification efficiency in the thermal-gradient deposition technique.