An image scanning apparatus with scan-starting point positioning function comprising a casing, a carriage and a calibration sheet is provided. Of which, the casing comprises a scan-platform used to place documents to be scanned, while the calibration sheet, which is fixed to the inner surface of the carriage and lies between the home line of the carriage and the upper wider margin of the scan-platform, comprises a hollowed-out mark completed through a punch-cutting manufacturing process. The mark can be defined using a predefined function set and has a reference point. After the carriage has moved to a scan line, the reference point can be located according to the intersection points between the mark scanned and the scan line accompanied by the predefined function set of the mark. Of which, the calibration sheet and the inner surface of the casing are of different levels of brightness.