The invention relates to a method for preparing active carbon by using a multi-hearth furnace. The method comprises the following steps: 1) inputting a raw material from the feeding end on the top of the multi-hearth furnace into the multi-hearth furnace; 2) starting a combustor in the bottom of the multi-hearth furnace to heat the raw material, wherein the temperature of the raw material in the furnace layer in the bottom of the multi-hearth furnace is raised to be not less than 600 DEG C in the first place to reach an activating temperature; 3) introducing water vapor into the multi-hearth furnace, wherein the raw material reaching the activating temperature and water vapor which are overturned by virtue of a rabble blade are uniformly contacted to carry out gas-solid heterogeneous activating reaction to generate combustible gases H2 and CO; 4) generating a negative pressure by using an induced draft fan to control the flowing directions of H2 and CO, wherein the H2 and CO move upward from the bottom of the multi-hearth furnace; 5) injecting air into the multi-hearth furnace, wherein exothermic reaction is carried out on H2 and CO and oxygen in air, and released heat is taken as heat required for carrying out gas-solid heterogeneous activating reaction on the raw material and water vapor; and 6) forming active carbon after activating reaction of the raw material, wherein the active carbon is discharged from a discharge opening in the bottom of the multi-hearth furnace. The method provided by the invention can be widely applied to processes for preparing active carbon.