The invention relates to a seawater COD automatic detector and belongs to the automatic detection technical field. The seawater COD automatic detector comprises a control device and a detect device, wherein, the control device is connected with the detect device and controls the detect device. The invention is characterized in that the detect device comprises a motor, a double plunger pump, a digestion device and a photoelectric sensor, a motor shaft is connected with the double plunger pump by a screw mandrel, the double plunger pump is respectively connected with a meter A and a meter B, which are connected with the seawater to be detected and a reagent pipeline through a pipeline and an electromagnetic valve and are connected with the digestion device through the pipeline and the electromagnetic valve, the digestion device is connected with the photoelectric sensor through the pipeline and the electromagnetic valve, the controller is connected with and controls the motor and the electromagnetic valve. The invention has the advantages of simple structure, convenient operation, reliable performance, stable motion, accurate and flexible detection, high detection efficiency, the integrated processes of the data processing, the data storing and the data transmitting, etc.