The disclosure provides a battery replacement mechanism configured to pick and place a battery from and in a first accommodation bay. The first accommodation bay is provided with a first latch to hold the battery therein. The battery replacement mechanism includes a multi-axial slide table assembly, a carrier, and a pick-and-place device. The carrier is movably disposed on the multi-axial slide table assembly, and the pick-and-place device is movably disposed on the carrier. The carrier includes a second latch, and the pick-and-place device includes a catching hook. The second latch is configured to release the first latch of the first accommodation bay, so that the battery may enter and exit the first accommodation bay, and the catching hook is connected to the battery and is configured to drag the battery toward and away from the carrier. The disclosure further provides a battery replacement system and a battery replacement method for a UAV.