1. A method for analyzing object motion in multi frames adapted to an image capturing device, comprising: obtaining a plurality set of first sum of absolute difference according to a noise of the image capturing device under a plurality of light settings, wherein each of the light settings respectively corresponds to a set of first sum of absolute difference; capturing a first frame and a second frame via the image capturing device under a picturing light setting of the light settings; calculating a plurality of second sum of absolute difference between the first frame and the second frame; finding a plurality of object block within a vein tracking of the object in the first frame and the second frame; calculating a local motion vector of each object block respectively according to the second sum of absolute difference; calculating a first reliability of each object block respectively according to the second sum of absolute difference and the set of first sum of absolute difference corresponding to the picturing light setting; and estimating the local motion vectors according to the first reliability to obtain a global motion vector.