The　pH　and　ionic　conductivity　of　anode　and　cathode　solutions　and　the　voltage　output　of　a　microbial　fuel　cell　(MFC)　were　tested　at　different　external　resistances,　and　the　results　were　analyzed　in　detail　from　the　perspective　of　mass　and　energy　in　this　paper.　Experimental　results　show　that　the　solution　pH　and　ionic　conductivity　changed　sharply　after　the　MFC　operated　with　small　external　resistances　for　a　long　period　of　time.　If　the　external　resistance　was　fixed　at　100　Ω,　the　maximum　pH　difference　between　the　cathode　and　anode　solution　was　0.82,　the　maximum　ionic　conductivity　difference　was　2.14　mS/cm,　and　the　corresponding　limiting　current　was　2.69　mA;　If　the　external　resistance　was　fixed　at　500　Ω,　the　maximum　pH　difference　was　reduced　to　0.41,　the　maximum　ionic　conductivity　difference　was　1.55　mS/cm,　and　the　corresponding　limiting　current　was　3.83　mA.　The　experimental　results　and　analysis　prove　that　for　small　external　resistances　reducing　the　changes　of　the　solution　pH　and　ionic　conductivity　during　the　MFC　operation　and　enhancing　the　mass　transfer　of　the　reactants　and　products　are　of　crucial　importance　for　improving　the　MFC　performance.