The invention discloses a method and a device for detecting chemical oxygen demand of a water body. The method comprises the following steps: applying 2.5V constant potential relative to a reference electrode to a boron-doped diamond film detection electrode as a working electrode by controlling an electrochemistry workstation; injecting certain volume of a sample to be detected into a background solution under a magnetic stirring condition; recording a current signal response of the working electrode acquired by the electrochemistry workstation in real-time; and calculating the chemical oxygen demand of the detected water body through current signals. The electrochemical detection method by the boron-doped diamond film detection electrode has the detection result in excellent comparable property compared with a national standard method, has high detection accuracy, wide the range without secondary pollution, and short electrode response time, ,and can meet the timely, quick and online requirement in detecting the current environment. Compared with Pbo2/Ti adopted in the current commercialized instrument, the detection signals have nearly one order of magnitude of predominance under the respective optimal use condition.