The invention provides a firmware extension method and a firmware builder, particularly adaptable for an embedded system. The firmware extension method comprises: providing a baseline firmware image and a header file correlated with the baseline firmware image; generating an extension firmware image according to the header file, wherein the extension firmware image comprises one or more extension functions; generating a callback table comprising pointers pointing to entries of the extended functions in the extension firmware image; merging the baseline firmware image, the extension firmware image and the callback table to generate a merged firmware builder. The firmware extension method and the firmware builder provided by the invention could extend the firmware without providing source codes of the baseline firmware image by the upstream supplier and could protect the source codes possibly relating to the firmware design correlated information.