1. An obstacle detection device, comprising: a storage unit at least storing an image; a quasi-vertical edge detection module coupled to the storage unit to receive an input image and perform a quasi-vertical edge detection procedure on the input image to obtain a plurality of quasi-vertical edges; an intersection determination module determining whether each of the quasi-vertical edges intersects a virtual horizontal served as a horizon line in the image and dividing the image into a above-horizon zone and a below-horizon zone; a position determination module determining whether an end of one of the quasi-vertical edges intersecting the virtual horizontal is located in a detection zone according to position of the end of the one of the quasi-vertical edges and range of the detection zone if the one of the quasi-vertical edges intersects the virtual horizontal, wherein the detection zone is located within the below-horizon zone which is below the virtual horizontal; and an obstacle detection module determining that an obstacle exists and issuing a warning if the end of the one of the quasi-vertical edges intersecting the virtual horizontal is located in the detection zone.