1. An object detection system comprising: a storage unit, storing at least a video sequence comprising a plurality of images; an image acquisition module, coupled to the storage unit, obtaining a first image corresponding to a first timepoint, a second image corresponding to a second timepoint, and a third image corresponding to a third timepoint of a video sequence from the storage unit, wherein a time difference between the first timepoint and the second timepoint is less than or equal to a time difference between the second timepoint and the third timepoint; a feature searching module, searching for at least one feature from the first image; a calculation module, determining whether the at least one feature is within a near-distance detected region or a far-distance detected region, calculating a motion vector of each of the at least one feature according to the first image and the second image when the at least one feature is within the near-distance detected region, and calculating the motion vector of each of the at least one feature according to the third image and one of the first image and the second image when the at least one feature is within the far-distance detected region; and a confirmation module, confirming an existence of a moving object according to the motion vector of each of the at least one feature.