Abstract: The humic acid mainly comes from the runoff input, its properties and composition are affected by its source. The indirect photodegradation behavior and mechanism of Diuron (DIU) in seawater were studied using four typical terrestrial humic acids as photosensitizers, and the effects of seawater environmental factors (salinity, pH and nitrate ions) on the indirect photodegradation of DIU were analyzed. The results showed that CDOM was the main participant in indirect photodegradation of DIU in seawater and it could significantly promote the indirect photodegradation of DIU. At the same dissolved organic carbon (DOC) mass concentration, JKHA promoted the indirect photodegradation of DIU significantly more than those of other three CDOMs. CDOM can generate ³CDOM^*, ·OH, ¹O₂ and other active intermediates under the light conditions, among which ³CDOM^* was the most effective reactive intermediates (RIs) that contributed 61% to indirect photodegradation of DIU. Four fluorescence components (C1～C4) were identified in the CDOMs used, and the fluorescence intensities of C1, C3 and C4 were significantly and positively correlated with the steady-state concentration of ³CDOM^* (correlation coefficients were all greater than 0.95), which was the main source of ³CDOM^* and played a major role in the indirect photodegradation of DIU. The relative intensities of terrestrial source humus fractions C1, C3 and C4 in JKHA were significantly higher than those of the other three CDOMs, resulting in a stronger promotion of indirect photodegradation of DIU by JKHA than by the other three humus fractions. The indirect photodegradation rate of DIU increased first and then decreased with the increase of salinity at salinity range from 0 to 35.0. In the range of pH from (7.00±0.10) to (9.00±0.10), the indirect photodegradation rate of DIU increased with the increase of pH, and its fastest degradation rate was found at pH (9.00±0.10). \mathrmNO_3^- could also promote the indirect photodegradation of DIU.