1. A method for image calibration comprising: S1: obtaining, by a processor, for each of photosensitive elements in a sensor, a high lighting parameter corresponding to a long exposure time and a low lighting parameter corresponding to a short exposure time; S2: obtaining, by the processor, image data of a sheet medium; and S3: calculating, by the processor, calibrated image data of the sheet medium based on the high lighting parameter, the low lighting parameter and the image data; wherein the process of obtaining, by the processor, for each of the photosensitive elements in the sensor, the high lighting parameter corresponding to the long exposure time comprises: performing a long time exposure on a gray scale board for a period of tH; and recording image data IHi obtained through the long time exposure and a corresponding gray value GrayH.