The principle of automatic color registration of flexible packaging bag gravure printing machine is as follows:The printing machine is run by the uncoiler and then passes through each printing unit, printing and drying of various colors, and is wound by the winder. Each color printing will be printed on the edge of the printing material with a color code for color registration. The marking lines of each adjacent color should be parallel to each other and vertically (longitudinal) when the registration is accurate.20mm. The photoelectric scanning head detects the color marks printed by two adjacent units. If the gap between adjacent two color scales is not equal to20mmit means that there is a deviation in overprinting, the deviation is calculated by the microprocessor, and the control signal is output to drive the actuator, so that the corresponding overprinting correction unit moves up and down to extend or shorten the printing plate of the unit on the printing material to the printing plate of this unit. The itinerary is dynamically corrected. The color registration of each unit is based on the color code printed by the previous unit.