Chemical oxygen demand (COD) is one of the most important parameters to assess pollution in seawater monitoring, which can be determined accurately by the combined use of UV-Vis spectroscopy and multivariate calibration. The calibration model of COD value was established by partial least squares (PLS). The COD of seawater determined by this method was very close to which determined by the standard method. Compared with the standards, this method has many different advantages, such as high-speed, sensitive, no preprocessing and no secondary pollution.