A method for generating a calibration curve is disclosed. The method comprises the following steps. Calibration data of one line image of a previous scan are first provided. Reference pixels are selected from the image line. Then a calibration board is scanned and calibration data of the reference pixels are generated. Calibration data of the reference pixels of a previous scan are then selected from the calibration data of one image line of a previous scan. Ratios of the calibration data of the reference pixels and the calibration data of the reference pixels of a previous scan are calculated. Calibration data of all pixels of the image line are finally generated by multiplying the calibration data of the image fine of a previous scan and the ratios.