An image sensor for a touch screen, which is installed at an edge of the touch screen and comprises an image sensing element and an optical imaging system, wherein the optical imaging system comprises: at least two separated lenses to constitute at least two optical paths, wherein each of the lenses includes a concavo-convex lens, an objective lens of each of the lenses is faced to the touch screen, an overlapping area of fields of the view of the objective lenses covers the touch screen, a touch object on the touch screen is imaged by each of the optical paths in the optical imaging system respectively, and the formed image is projected on the image sensing element. The image sensor can locate the touch object by utilizing stereo photography. Moreover, the optical imaging system can utilize the interlocked electronic shutters to realize time division multiplex of the image sensing element by the different optical paths, or utilize individual optical paths to realize area division multiplex of the image sensing element.