The invention discloses a device based on a microorganism electrolytic tank technique for on-line biochemical oxygen demand measurement. The device comprises a microorganism electrolytic tank, a constant potential meter, connecting tubes, a liquid conveying pump, a hydrocyclone, an automatic sample diluter, a static vortex mixer, an on-line degassing machine, a liquid storage tank, an incubator, a computer and a control system, wherein the computer and the control system are respectively connected with a microorganism electrolytic tank sensor; the microorganism electrolytic tank is connected with the liquid conveying pump, the hydrocyclone, the automatic sample diluter, the static vortex mixer, the on-line degassing machine and the liquid storage tank through the connecting tubes. As the influence of oxygen is eliminated, the device disclosed by the invention has the advantages of high sensitivity, short detection time, wide linear range, low limit of concentration detection, simplicity in operation and the like, the biochemical oxygen demand can be measured on line, and the monitoring level is greatly improved.