To accurately measure the Fe3+ concentration existing in microbial fuel cell (MFC), a plastic optical fiber (POF) evanescent wave sensor with D-shape structure was put forward. The Fe3+ indicator sensitive film was deposited on the surface of POF through layer-by-layer(L-b-L) electrostatic self-assembly technique as indicator to the concentration of Fe3+. Experimental results illustrate that the sensitivity and response time are related to the number of sensitive film layers and the content of indicator. When the layer number is 20, the response time is about 13min, and the minimum detection limit hits 10-6mol·L-1. The results show that the sensor has good optical response to Fe3+ aqueous solution.