An apparatus and method are provided for detecting a scanning boundary of a scanner. The apparatus includes a reference unit, an image-pickup unit, and an operating process unit. The detecting method includes steps of: a) providing an initial optical signal; b) receiving the initial optical signal and generating a reference signal in response to the initial optical signal; c) setting a first reference pixel in the scanner; d) receiving the reference signal and generating a second reference pixel; e) calculating a difference between the first reference pixel and the second reference pixel; and f) detecting the scanning boundary after adjusting the scanner according to the difference.