The invention relates to a multifunctional solid state fermentation reactor. The reactor comprises a main tank, an auxiliary system and a control center, wherein a tank cover is fixed at the upper port of the main tank through a flange; a cylinder port is fixed on the top of the tank cover; the upper end face of the cylinder port is provided with four ports; pipelines connected with the four ports are connected with a gas purification module, a pressure meter and a gas analysis module through valves; the tail end of the pipeline of a first port is connected with a four-way pipe; and one port of the four-way pipe is connected with the main tank through a gas pipeline, and the formed gas forms a loop from a fourth port to a fifth port; and the auxiliary system consists of an aseptic inoculation part, a circulating gas part and a foreign gas part. The reactor can realize a closed solid state fermentation process without mixed bacterial pollution or pressure pulsation, can be applied to the process of quickly decomposing solid organic matters, and can monitor various parameters in the fermentation process, and a method for extracting a required fermentation product in the reactor in situ is provided.