Aiming at the problems of long detection time, inconvenient operation and difficult online detection of traditional measurement methods of chemical oxygen demand (COD), a COD detector based on sequential injection analysis (SIA) and image processing technology is designed. The core of the system is the principle design of sequential injection image analysis platform. Image processing process design, digestion and detection process design, digestion tank and structure design of the detection pool. Experimental research on the new method of SIA combined with image processing and analysis technology to determine COD is carried out. Based on the national standard water quality detection standard, the sequential injection image analysis and detection process is improved, and after the water sample is taken, the potassium dichromate and silver sulfate are added directly to digestion, and then the image is collected after cooling. Based on the partial least squares regression of the single variable, the regression model of COD concentration on the characteristic values of image color Red(R), Green(G), Blue(B), Hue（H), Saturation(S) and Intensity(I) is established, and the regression equation is obtained. The multiple coefficient of determination R2=0995 1, and the root mean square error RMSE=4937 3. The experimental results showed that the minimum detection limit was 20 mg·L-1, and there was no significant difference in repeatability (RSD≤438%) compared with the national standard method. The system can analyze COD content in different water samples stably and efficiently. The method of COD determination based on SIA combined with image processing is helpful to improve the technical performance of water quality testing instrument and is suitable for many kinds of water quality testing parameters.