The invention provides a method for directly growing a carbon nano tube array on a fiber substrate. The method comprises the following steps: (1) pre-treating the fiber substrate; (2) attaching a catalyst precursor on the surface of the substrate obtained in the step 1, wherein the catalyst comprises transitional metal elements such as Fe, Co, Ni, Cu, Au, Pt, Mo or Ag; (3) putting the substrate obtained in the step (2) in a reaction furnace, heating the substrate to a first temperature and maintaining for a preset time, and introducing inert gas to remove organic matters in the catalyst precursor; (4) heating to a second temperature and maintaining for a preset time, continuously introducing inert gas, introducing mixed gas of reductive gas and carbon source gas, maintaining the pressure in the reaction furnace, and growing high-density carbon nano tubes on the surface of the substrate; and (5) after reaction, stopping introduction of the reductive gas and carbon source gas, and continuously introducing inert gas till the temperature is reduced to room temperature.