A method for defining effective pixels in an image sensing array of a light-shading optical touch system is proposed. First, light-shading objects are disposed at three end points of a touch panel, and a shaded image is generated by an image sensing array. Light-shading object pixels corresponding to the light-shading objects are obtained from the shaded image thereafter. Next, the light-shading objects are removed from the touch panel, and a non-shaded image is generated by the image sensing array. Three pixel columns corresponding to the end points are obtained from the non-shaded image, and a pixel with maximum intensity is obtained from each of the three pixel columns. Interpolated pixels between the three brightest pixels are obtained through interpolation from the non-shaded image. Image sensing pixels corresponding to the three brightest pixels and the interpolated pixels in the image sensing array are defined as effective pixels.