Bioleach deep dehydrated sludge was used as the main material, four kinds of agricultural and forestry organic wastes were used as conditioners for mixed composting. Four treatment groups (T1: sludge+bagasse, T2: sludge+straw, T3: sludge+rice bran, T4: sludge+sawdust) were set for mixed composting. UV-vis spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), and three-dimensional fluorescence spectrum (3D-EEM) were used to study the structural characteristics and component content evolution of dissolved organic matter (DOM) in co-composting. The UV-vis results showed that the aromaticity and unsaturation of DOM increased in all treatment groups, and the T3 treatment group showed the largest increase. SUVA₂₅₄ and SUVA₂₈₀ of four composting treatment groups showed an increasing trend. The change range of the T3 treatment group was higher than the other three treatment groups, indicating that the degree of aromatization deepened, the molecular weight of DOM gradually increased. E₂₅₃/E₂₀₃ and E₂₅₃/E₂₂₀ increased significantly at the end of composting, indicating that the aliphatic chain on the benzene ring in DOM was oxidized, decomposed, and transformed into functional groups such as carboxyl group and carbonyl group. A_226~400 increased with composting, whileE₂₅₀/E₃₆₅ decreased, indicating that the conjugation degree increased. FTIR results show that the content of polysaccharides and aliphatic substances decreased during composting, and unsaturated organic substances such as aromatic compounds increased. The transformation degree of the T4 treatment group was better than the other three treatment groups. Emission fluorescence spectra showed that the fluorescence peak position shifted from 334nm to around 422nm with composting, which indicated that the substances with low conjugation degree were continuously degraded, and aromatic groups were continuously condensed to form humic-like matters. In the synchronous fluorescence spectrum, with composting time, the fluorescence peak of protein-like substances changed from strong to weak, the fluorescence peak of humic-like matters changed from weak to strong, A_250~308 decreased, A_308~360 and A_363~500 increased, which also indicated that protein-like substances were degrading, while humic acid-like substances and fulvic acid-like substances increased. Combined with the parallel factor (PARAFAC) model to analyze the three-dimensional fluorescence spectrum, DOM was divided into three components. According to the analysis and judgment of the excitation and emission wavelength positions, the three components are fulvic acid-like substances, humic acid-like substances and tryptophan-like substances. The percentage of C1 (fulvic acid-like substances) and C2 (humic acid-like substances) components showed an increasing trend, while the percentage of C3 (tryptophan-like substances) components showed a decreasing trend, indicating that protein-like substances decreased. At the same time, humic-like matters increased, and the humification degree of T3 and T4 treatment groups were good. Comprehensive analysis showed that rice bran and sawdust as conditioners had better compost maturity.