The invention relates to device and method for depositing an ultrathin alumina film by an atomic layer. Plasma is generated by combining double frequency which is microwave ECR with radio frequency negative bias voltage equipment and used for depositing the alumina film. By using the microwave ECR and the radio frequency negative bias voltage system, the work gas generates the plasma; and by using trimethyl aluminum (TMA) as a monomer and the mode of TMA-Ar-O2-Ar alternate pulse, the alumina film is deposited by using the atomic layer, wherein the deposition temperature is room temperature. By adopting the generating mode of using the plasma as an active group, the deposition can be carried out in a lower ambient temperature, the growth speed is higher compared with the deposition by a hot atomic layer and is about 0.12nm/period and the quality of the prepared film is higher compared with the deposition by the hot atomic layer. The prepared Al2O3 film can be widely applied to the fields of microelectronic devices, electroluminescent devices, wave optical devices, corrosion-resistant coatings and the like.