An automatic image calibration method is provided for a contact type scanner. The inventive method involves in first reading an image of a test chart that consists of at least a horizontal line and a plurality of slanting lines. Then, compute the calibration parameters according to the image of the test chart. The calibration parameters include a vertical displacement value, a horizontal displacement value, and a slope value. After obtaining the calibration parameters for each sensor chip, the initialization step is complete. The calibration parameters must be stored in a memory device for an application program to read. Later on, for each image read by the contact type scanner, the application program can automatically calibrate the image according to the calibration parameters stored in the memory device.