1. An optical touch device comprising: an indication plane having a first edge and a second edge, the first edge being opposite to the second edge; a first image sensing unit and a second image sensing unit separately disposed on the first edge; a third image sensing unit and a fourth image sensing unit separately disposed on the second edge, the first image sensing unit being opposite to the fourth image sensing unit, the second image sensing unit being opposite to the third image sensing unit, a central touch area being defined between the first image sensing unit, the second image sensing unit, the third image sensing unit and the fourth image sensing unit; and a processing unit electrically connected to the first image sensing unit, the second image sensing unit, the third image sensing unit and the fourth image sensing unit; wherein when a touch gesture is performed on the central touch area, the first image sensing unit senses a first image, the second image sensing unit senses a second image, the third image sensing unit senses a third image and the fourth image sensing unit senses a fourth image; the processing unit calculates a first coordinate of a touch point according to the first image and the fourth image, calculates a second coordinate of the touch point according to the second image and the third image, and integrates the first coordinate and the second coordinate with each other by a weighting, so as to calculate an output coordinate of the touch point.