The invention discloses a universal main board for reducing the number of air conditioner external unit main board types and a programming method of the universal main board. The programming method comprises the steps of writing an EEPROM testing program on an EEPROM chip of the universal main board, and conducting performance test on the universal main board; writing a corresponding overall unit EEPROM program on the EEPROM chip according to the type parameters of an air conditioner; affixing the name identification of the overall unit EEPROM program on the universal main board. Multiple types of main boards corresponding to different parameters can be replaced only through arranging one type of main board, and the overall unit functions can be achieved through writing the corresponding overall unit EEPROM program on the universal main board; accordingly, the number of the air conditioner external unit main board types is decreased greatly.