As the key parameters of marine water quality monitoring, nitrate and nitrite are important references for studying the marine nitrogen cycle. The ultraviolet (UV) absorption spectrum is simple to operate and fast in response, and it is suitable for monitoring various water quality parameters. However, the measurement has aliasing interference between nitrate and nitrite and seawater ion interference. This paper proposes a rapid measurement method for nitrate and nitrite. First, the spectrum is preprocessed by first-order difference. Then, the correlation coefficient search is used to obtain the optimal modeling band. Finally, the BP neural network is used for concentration inversion. The linear correlation coefficient between the predicted value and the real value of the model is 0.998. The comparison test was conducted with similar equipment, and the linear correlation coefficient was 0.982. The application test was carried out in the 2023 summer voyage of Yantai Marine Environment Monitoring and Forecasting Center, and the linear correlation coefficient between the two methods was 0.902. The results show that this method can realize the rapid measurement of nitrate and nitrite in seawater and provide a reference for developing subsequent in-situ monitoring instruments.