Dissolved organic matter (DOM) was extracted from rice and wheat straw at different decomposition phases in aquatic systems. The component, structure and photochemical activity of the DOM were characterized by UV-vis absorption spectroscopy, three-dimensional fluorescence spectroscopy and static photochemical reaction methods. Results showed that decomposition process of straw concluded three phases, i.e., physical-leaching, vulnerable and refractory component decomposition, and the vulnerable decomposition component was the main source of the DOM. The aromaticity, humification degree and molecular weight of the straw-derived DOM increased as a function of decomposition time, while the bioavailability of the DOM gradually decreased. The tyrosine-, humic acid- and fulvic acid-like substances of the DOM gradually accumulated during the decomposition period. By the end of decomposition, the contents of the three components in rice and wheat straw-derived DOM increased by 4.2%~14.3% and 5.9%~12.8%, respectively. However, the tryptophan-like substance and soluble microbial secretion were relatively unstable and gradually decomposed. The UV and fluorescence spectral indices SUVA₂₅₄, E2/E3, S_275~295, S_R, BIX and FI were strongly correlated to the photochemically generated HO×, ¹O₂ and triplet-state DOM (r > 0.61, P < 0.05). Therefore, the photochemical activity of the DOM was concurrently dominated by its aromaticity, molecular weight and bioavailability. Based on the results, it is suggested that exploring photochemical activity of bioavailable components and constructing mathematical models for predicting photochemical activity of DOM should be strengthened in the future studies, which is helpful for understanding the ecological significance of straw-derived DOM.