Microbial fuel cells (MFCs) are devices that use microorganisms as the catalysts to oxidize organic and inorganic matter and generate current. Previous researches indicate that the internal resistance is an important factor to limit the power output of MFCs. So, the researches on the internal resistance of MFCs at home and abroad were summed up and the definition, constitution and the common determine methods of the internal resistance are systematically introduced in this review. The influencing factors of MFCs’ internal resistance were analyzed in detail in terms of reactor configuration, substrates, microorganisms and operating conditions. Based on the previous results, the feasible methods are proposed to reduce internal resistance and improve power output of MFCs.