Abstract: A quantitative model for detecting the concentration of dissolved nitrate in seawater was established based on Ultraviolet spectrophotometry combined with partial least squares (PLS) method.The optimal wavelength range was determined using interval PLS (iPLS) method.Compared with the model used full spectrum, the model used the optimal wavelength range determined by iPLS method lowered the RMSECV of the training set from 39.1 to 9.83.The test set including 35 samples were used to verify the effect of iPLS method, the results showed that the iPLS method lowered the average relative deviation of the test set from 3.23% to 1.81%.The study showed that the optimal wavelength range determined by iPLS not only reduced the number of independent variables, simplified computing, but also improved the prediction accuracy.