An air extracting device (2) at least includes a processor (21), a motor (23), an air-extracting unit (24), and a wireless transmitting unit (25), and a method of the air extracting device (2) for calculating remaining time required for extracting action includes following steps: activating the motor (23) after the air extracting device (2) and a sealed bag (1) are jointed together; driving the air-extracting unit (24) to execute an extracting action by the activated motor (23) for extracting air from the sealed bag (1); monitoring load current of the motor (23) by the processor (21) while executing the extracting action; calculating a remaining time required for the extracting action by the processor (21) according to a variation of the load current; and, transmitting the remaining time required for the extracting action to a mobile device (4) through the wireless transmitting unit (25) for displaying thereon.