The chemical composition and biogeochemical reactivity of DOM in the lower Minjiang River-Estuarine system were examined with absorption spectroscopy and fluorescence EEM-PARAFAC. The annual export of DOC from the Minjiang River was 5.48×1010 g yr-1, which was a largest river DOC flux adjacent to the Taiwan Strait. The freshwater absorption coefficient a280 and DOC-specific UV absorbance SUVA254 were much higher, while the spectral slope S275-295 was lower in summer than in winter. This indicated stronger flushing of aromatic and high molecular weight constituents from the watershed in the rainy season. Six fluorescent components were identified using EEM-PARAFAC, including three humic-like components C1-C3, two tryptophan-like components C4 and C6, and one tyrosine-like component C5. The levels of four components (C1, C2, C4, and C6) in the freshwater were lower while that of C5 was higher in the wet season than in the dry season, suggesting contrasting seasonal variations of different DOM constituents. Photochemical degradation removed effectively CDOM and FDOM, which was stronger for large molecular weight constituents as indicated by an increase in S275-295 and for summer under stronger solar radiation than for winter. Microbial degradation generally showed little effect on DOC, and it had smaller impact on CDOM and FDOM in winter than in summer which might be partly related to lower temperature in winter. Overall, this study demonstrated evident seasonal changes in the chemical composition of DOM and different PARAFAC components. The results also showed the seasonal variability of DOM reactivity, and emphasized the significant effects of photo-degradation on the DOM characteristics.