A firmware extension method is provided, particularly adaptable for an embedded system. Firstly, a baseline firmware image is provided with a header file. Secondly, an extension firmware image is generated based on the header file, comprising one or more extended functions. Thereafter, a callback table is generated, comprising pointers pointing to entries of the extended functions in the extension firmware image. The baseline firmware image, the extension firmware image and the callback table are then merged to generate a merged firmware image.