The invention discloses an apparatus and method for evaluating congestion condition of a constructed wetland based on microbial fuel battery, and belongs to the technical field of environment monitoring. The apparatus includes a constructed wetland sludge reservoir, a coupled microbial fuel cell and a voltage test circuit. The coupled microbial fuel battery is provided successively from top to bottom with a cathode region, a first filter material region, an anode region, a second filter material region and a supporting layer, wherein the first filter material region and the second filter material region are separately and at least provided with sludge inlet opening. The constructed sludge reservoir is connected to the sludge inlet opening through a material inlet pipe which is provided with a sludge pump thereon. The diluted sludge is discharged to the coupled microbial fuel cell through the sludge inlet opening. The voltage test circuit includes a titanium wire which connects the cathode and the anode, an external resistor which is arranged on the titanium wire, and a data recorder which is in parallel connection to the external resistor, and the data recorder records the voltage changes of the external resistor per minute, and the congestion of the constructed wetland can be evaluated.