A shutdown controlling method is applied to a system that has a host computer, a PDU and multiple electric apparatus connected to the PDU. The PDU has multiple outlets and stores preset shutdown periods for the outlets respectively. The PDU communicates with the electric apparatus and obtains their required shutdown periods. When PDU is controlled to cut off an outlet, the PDU determines whether the preset shutdown period of the outlet is larger than the required shutdown period. If yes, the PDU cuts out the AC power of the outlet. If not, the PDU cancels the shutdown command and notifies the host computer. Therefore, a manager may update the preset shutdown period of the outlet to ensure that each electric apparatus has enough time to be safely shut down.