The present disclosure relates to a scanning method for an electronic control unit in a vehicle and a vehicle diagnostic device. The vehicle diagnostic device is communicatively connected to at least two vehicle buses in the vehicle, each of the vehicle buses is connected to at least one electronic control unit in the vehicle. The scanning method determines a scan list corresponding to each of the vehicle buses. The scan list includes a to-be-scanned electronic control unit. The vehicle diagnostic device transmitting a scan signal on the vehicle buses in parallel according to the scan list corresponding to each of the vehicle buses, to determine whether the to-be-scanned electronic control unit in the scan list exists in the vehicle. The parallel scanning in this way can effectively improve the efficiency of automatic scanning and significantly shorten a time required to complete the automatic scanning, and improves user experience.