1. A coordinate determining system for determining a screen coordinate (X,Y) of an indicating point shown on a screen, the coordinate determining system comprising: an image capturing module for capturing a plurality of images corresponding to the screen and a dividing module coupled to the image capturing module for dividing an overlapping image region of the images into a target image comprising (M*N) sub-regions, wherein a plurality of vertical bars and horizontal bars are projected on the screen at different time respectively by the dividing module, the plurality of images comprise one of the vertical bars or one of the horizontal bars respectively; a determining module, coupled to the dividing module, for determining a position of the indicating point on the target image comprising the (M*N) sub-regions; a positioning module, coupled to the dividing module and the determining module, for positioning a plurality of grid points formed by the sub-regions according to a plurality of fiducial points, the determining module determining a target sub-region according to a plurality of line equations formed by the grid points, wherein the positioning module adjusts the plurality of fiducial points according to an adjusting vector, wherein a reference point is generated from a parallelogram determined by the corner grid point of the target image and its neighboring grid points such that the adjusting vector is generated by connecting the reference point and the corner grid point, and a new fiducial point is generated by subtracting the adjusting vector from the corner grid point; and a converting module, coupled to the determining module, if the position of the indicating point is in the target sub-region (a,b) among the (M*N) sub-regions and has an image coordinate (x,y) relative to an origin of the target sub-region (a,b), the converting module converting the image coordinate (x,y) into the screen coordinate (X,Y) according to a proportional parameter, wherein a is a positive integer less than M, and b is a positive integer less than N.