The rape and rice straws were collected in the West Sichuan Plain. The spectral characteristics of dissolved organic matter (DOM) released from the decomposing process of crop straws were characterized by the UV-Vis spectrum, Fourier transform infrared spectrum (FTIR) and three-dimensional excitation emission matrix fluorescence spectrum (3D-EEM). The difference of spectral characteristics of DOM in rapid leaching stage (0～0.5 days) and straw decomposing process (0.5～90 days) was clarified. The effect of straws to ecosystem structure in the conservation tillage pattern was tracked, and the variation mechanism was analyzed. The difference of DOM spectral characteristics in decomposing processes was significant. The dissolution of aromatic substances, degradation of components and formation of humus also presented in the processes. The E2/E3 value of DOM released from the rapid leaching stage of the straw was the highest, while the value of SUVA254 was the lowest, which suggested the aromaticity and molecular weight of DOM at the stage were relatively lower than those in other periods. The dissolution of aromatic components predominantly occurred in the early stage (≤10 days) when the E2/E3 value decreased gradually, but the SUVA254 value increased. And, the peak of polysaccharide, aromatic carbon and amide absorbent apex in DOM weakened. However, the persistent cellulose, hemicellulose and macromolecular protein were decomposed in the later period. The IR bands of DOM consisted of the amino acids, peptides, proteins, polysaccharides, phosphate skeleton, purine bases, pyrimidine and other components were mainly caused by the vibration, stretching, bending or hydrogen bonding of N—H, O—H, CO, CC, C—O, C—H and —COO- functional groups. Microorganism activities were vigorous in the early and middle stages of decomposition, which made degradable sugar and protein-like rapidly running out. DOM released from rape straws in the early stage showed a high bioavailability, due to the protein-like with the low molecular weight and unstable characteristics. The DOM of rice straws contained more organic matter than the rape, and had more humus relatively. The —OH and CO of DOM can disturb the migration and transformation of non-ionic polar OCs through covalent bonds in the soil. With the decomposition, —OH was, to a certain extent, decomposed by microorganisms. In this case, the interaction between DOM and OCs became weaker, and the phenols, carbonyls and carboxylic acids gradually enhanced the interaction between DOM and OCs or metal ions. Therefore, DOM regulated the migration and transfer processes of OCs in farmland soils. This study provides the basic data and new evidence for environmental significance of straw-return in the West Sichuan Plain, and the support for agricultural waste treatment and resource recycle in the area.