Acute toxicity assay by using microbes as bioindicators is an effective tool for reflecting the impact of water pollution on organisms. Until now, the suspended state of microbes is still mainly adopted, because the immobilized state of microbes will cause the poor stability of toxicity assay due to the microbes being reused. Here, an electrochemical biosensor based on biofilm was established for toxicity assay (TOX-biosensor), and the stress response of immobilized microbes in biofilm was demonstrated as the main factor that resulting in instability of the TOX-biosensor, and it can be induced by both of dose and time accumulation. The intensity of stress response for measuring a toxicant absence of organic substance was triple of that presence, indicating extracellular substances can be used for microbial repair and result in the decrease of the consumption of endogenous substance. Herein, a weak magnetic field (WMF) was employed to balance the activity of biofilm in the present TOX-biosensor, and a low intensity of WMF 10 GS was demonstrated to have the function for weakening stress response. Under this condition, the signals of biosensor can be kept stable for a month and the recovery time of dormant TOX-biosensor can be shorted from 50 h to 10 h. Its mechanism may be associated with the special magnetic field inside the organisms and the charge transfer happened in the life activity process.