In this study, the authors investigated the electronic structures of natural goethite FeOOH by first principle theory and synchrotron soft X-ray emission and absorption spectroscopy. The results show that the band gap of natural goethite FeOOH is 2.30 eV, smaller than the band gap of pure goethite FeOOH (2.57 eV). Due to the complex compositions and plenty of defects, the band structure of natural minerals is hard to measure by traditional methods, such as UV-Vis. The results supply a new way for determining the band structure of natural semiconductor minerals and also provide a basis for better understanding of photocatalytic mechanism of them under visible light.