The pollution of excessive sewage of oil production is harmful to the soil environment and aquatic resources. It is necessary to monitor the water quality of oilfield, indentify the pollutant index and reduce the pollution of environment simultaneously. Based on transmission and scattering theory of near-infrared light, a measurement model was proposed to test oil and suspended matter concentration in sewage at the same time. An appropriate light wavelength was selected, which was employed to realize the photoelectric detection using three-wavelength method. Furthermore, a plug-in online monitoring system was developed to carry out the experiments. The experimental results show that the proposed method has the advantages of high accuracy and reliability, and the developed system can simultaneously detect the components of oil and suspended matter in the sewage, and the deviation error of the system is less than 6% in comparison with the conventional method of infrared luminosity to detect the components of oil and the weighting method to detect the suspended matter in sewage. Therefore, the proposed method provides great reference for application in practice.