The invention discloses a microbial fuel cell toxicity sensor and an operation method and belongs to the technical field of new energy and environmental engineering. A reaction device is a double-chamber MFC and is partitioned by a cation exchange membrane; a cathode chamber contains a nutrient solution and is used for aeration; meanwhile, the cathode chamber communicates with a nutrient solution tank through a peristaltic pump and forms a circulating pipe; an anode chamber contains heavy metal wastewater; the anode chamber communicates with a heavy metal wastewater tank through the peristaltic pump and forms a circulating pipe; a carbon brush is adopted as an anode material of the anode chamber and a carbon felt is adopted as a cathode material of the cathode chamber; a cathode and an anode are connected together through an additional resistor and two ends of the additional resistor are connected with voltage data collection systems; and toxicity detection can be carried out after the voltage is stable under the optimal condition. The cathode chamber is used for automatically preparing a nutrient solution in cycle and carrying out aeration; and the anode chamber is used for circulating heavy metal-containing wastewater. Detection is carried out from the minimum concentration of a toxic substance, and the removal rate can reach over 90% when the voltage is stable after operation for 12h.