An automatic diagnosis method implemented by an automatic diagnosis system is provided. The automatic diagnosis system has a network device and a portable storage device. The method has the following steps of: detecting whether the portable storage device is plugged into a transmission interface of the network device; when it is detected that the portable storage device is plugged into the transmission interface of the network device, a system program is executed via the network device, so that the system program loads and executes a diagnosis program stored in the portable storage device; analyzing a network condition of the network device by the diagnosis program to generate a status report; and writing the generated status report to the portable storage device.