The invention provides a method for extracting metallic aluminium from aluminum oxide by vacuum carbothermal reduction, which comprises the following steps: adopting aluminium oxide or minerals containing aluminium oxide as raw material, auxiliarily taking graphite as reducing agent and aluminium trichloride as chlorinating agent, carrying out carbothermal reaction in a vacuum furnace under the condition that the system pressure is 20Pa, the reducing temperature is 1400-1600 DEG C and the reaction time is 60-90min, then carrying out chlorination to carbothermal residue for 30-60min and finallyobtaining aluminum subchloride (AlCl), when temperature lowers, disproportioning and decomposing AlCl into metallic aluminium and aluminium trichloride at the temperature less than 660 DEG C, and obtaining metallic aluminium with purity more than 95% and recovery efficiency reaching more than 80%. The recovery efficiency of aluminium trichloride is more than 80%; and the process flows are short,the cost is low and the pollution to environment is small.