The invention relates to a water quality online security early warning system and method thereof. The water quality online security early warning system comprises a biology avoidance behavior sensor which consists of two testing tubes, wherein, the two tubes are communicated through a two-way in the middle and parallel with each other; the two tubes are respectively provided with two groups of stainless steel monitoring electrodes, each group of electrodes consists of a signal transmitting electrode and a signal receiving electrode, wherein, the signal transmitting electrode forms a positive rotation low voltage alternating electric field in the testing tube, and the signal receiving electrode can induce changes of the electric field; the changes of the electric field are transmitted to a signal collecting device, and displayed through a display after being processed. The invention can display behavior change signal of a tested biology with the mode of electrical signal received from multi-point monitoring, can achieve online biology monitoring of original position, online and real time via analyzing and evaluating the electrical signal, and can warn early against unknown contaminants in water body, and has very important indicating effect for the investigations of environmental pollution, in particular for paroxysmal and severe accidental changes.