An electrogen strain Klebsiella oxytoca-d7 was isolated from biofilm on the anode of a stable operating double chamber microbial fuel cell (MFC) using anaerobic double-plate method and its electricity-generating mechanism was investigated in this study. The results show that the bacteria Klebsiella oxytoca d7 performed high electrochemical activity according to the cyclic voltammetry analysis. It carried out extracellular electricigenic respiration in the environment with extracellular electron acceptor. The strain accomplished electricity generating respiration mainly through humus secreted as extracellular electron media detected by the three simensional fluorescence analysis of the strain's metabolite. Ferric reducing experiments implied that excessive ferric ion did not result in extra growth of the strain, and the ferric ion reduction rate of the strain was limited. This research suggests that extracellular respiration is not the mainly respiration path way of the Klebsiella oxytoca d7 under anaerobic condition, but a subsidiary one as of the intracellular respiration in the presence of extracellular electron acceptor in the environment. Even there are enough extracellular electron acceptor in the environment, the strain's growth won't increase significantly. It is considered the strain does not obtain main growth energy from extracellular reduction, but from intracellular respiration. Only a small part of the electrons produced by oxidation of organic is used to reduce the extracellular electron acceptor.