Program upgrading method for main ECU (Electronic Control Unit) and sub-ECU, main ECU and system

The invention relates to the field of software upgrading, and discloses a program upgrading method for a main ECU and a sub-ECU, the main ECU and a system. The method comprises the following steps: the main ECU receives a diagnosis message transmitted by an upper computer, and judges a mode corresponding to a preset unvarnished transmission identifier; if the unvarnished transmission identifier corresponds to a main ECU mode, the main ECU sends a response message including a second diagnosis ID to the upper computer through a public CAN network between the main ECU and the upper computer, and program upgrading is carried out; and if the unvarnished transmission identifier corresponds to a sub-ECU mode, the main ECU forwards the diagnosis message to the sub-ECU through a private CAN network between the main ECU and the sub-ECU, so that the sub-ECU performs program upgrading. According to the embodiment of the invention, the program upgrading of the main ECU and the sub-ECU is realized by sharing one pair of di