A movable carrier auxiliary system includes a sign detecting device including an image capturing module, a storage module, and an operation module. The image capturing module has at least two lenses having refractive power for capturing an environment image around a movable carrier. The storage module stores a plurality of sign models. The operation module is electrically connected to the image capturing module and the storage module. A processing method of the movable carrier auxiliary system includes steps of: detect whether the environment image has a sign image that matches at least one of the image models or not, and correspondingly generate a detection signal via the operation module, thereby to identify signs around the movable carrier.