With the rapid development of offshore oil exploration and development technology, an increasing amount of production water needs to be treated by water treatment facilities before being discharged into the sea or reinjected, posing a serious threat to marine environmental safety. Establishing a comprehensive optical in-situ monitoring technology for pollutants discharged from offshore oil platforms is of great significance for promoting the development of smart oil fields and ensuring marine ecological security, in order to facilitate efficient exploration and development of marine oil and gas. Although multiple core technologies have been breakthrough in the field of marine environmental monitoring sensors, online monitoring of oil in water still mainly remains in the stage of laboratory prototype verification and theoretical exploration, and has not yet fully adapted to the special requirements of harsh working conditions on offshore platforms. Two types of oil sensors in water were designed based on ultraviolet fluorescence method, with measurement accuracy better than 0.1 mg/L in the range of 0~100 mg/L. The linear fitting coefficient of the sensors reached R²>0.97.The relative stability tests for different concentrations all meet RSD<0.5%, which meets the requirements for offshore field applications. Afterwards, an online monitoring system for oil content in the production water of offshore oil platforms was built and sea trials were completed. During the experiment, the online monitoring system for oil in water achieved 24-hour continuous acquisition of oil content data in the production water discharged from offshore platforms. The experimental results show that the developed fluorescence based oil monitoring system in water can accurately reflect changes in oil concentration in water, and has fast response capability and high concentration detection range for substances in water. Considering that the transportation of platform water samples back to the onshore laboratory for testing does not have real-time capability, and the water samples may undergo physical and chemical phenomena such as sedimentation and dissolution, and can not reflect changes in oil concentration in the production water over a period of time, a one month online monitoring and laboratory manual testing method comparison test was conducted at the offshore site. The results indicate that the two methods have a high degree of trend consistency, with test data reaching 98.5% within the standard range. This suggests that using fluorescence method for on-site online monitoring has the characteristics of simple operation and high accuracy, meeting the requirements of offshore field applications. This method will greatly improve the efficiency and data accuracy of water monitoring for offshore platforms, and also provide reliable data support for the unmanned operation of offshore platforms. This research and development plan is selected as a demonstration application area for a certain platform in the East China Sea, which has the same applicability for different sea areas and oil fields. In the later stage, a large sample range will be used to conduct relevant experiments on offshore platforms in multiple regions to verify the feasibility of fluorescence based online detection of oil concentration in water. This technology will also serve as a key support for the construction of smart oil fields, ensuring real-time protection of marine ecological security.