The invention provides a method for automatically managing a large number of devices based on an EMS. The method is characterized by that MO (Monitor Object) information supported by devices is obtained by updating monitor information lists of local EU (Extended Unit) devices; MOs related to in-place serial numbers and in-place states of RU (Remote Unit) devices are extracted from the MO information; and through combination of a preset MO mapping rules of the EUs and the RUs, the in-place RU devices are automatically virtualized, and the MO information is mapped to the virtual RU devices. According to the method, slave device RUs are automatically managed through master device EUs, a traditional complex and error-prone working mode of artificially extracting reports and recording information when the devices are increased and deleted is changed, the artificial maintenance cost is greatly reduced, the working efficiency and reliability are improved, and demands of operators for unattended operation, reduction of cost and improvement of efficiency are satisfied.