The invention relates to a biochemical oxygen demand BOD online measuring instrument and a detection method thereof. The sewage pump, dilution water pump, air pump and buffer pump are connected to the aeration tank, the aeration tank is connected to the reactor, the reactor is connected to the controller, the connecting pipe and the connecting wire are equipped with dissolved oxygen electrodes, and the controller is connected to the sewage pump, dilution water pump. When the sewage to be tested passes through the sewage pump, the dilution water passes through the dilution water pump, and the air is pumped to the aeration tank through the air pump to aerate and reach dissolved oxygen saturation, after passing through the porous ceramic of the reactor, measure the change of dissolved oxygen concentration before and after passing through the porous ceramic material . The dissolved oxygen concentration of the effluent sewage after the reaction is continuously monitored by the dissolved oxygen sensor. The controller controls the input flow rate of the sample to keep the change of the dissolved oxygen concentration constant, and calculates the BOD value according to the flow rate of the sample to be tested under the condition that the dissolved oxygen concentration reaches the same constant value after the reaction and the input flow rate of the standard sample. The measurement process of the present invention is fully automated, can monitor continuously in real time, and display the dynamic BOD value of the water body at any time.