Taking black soil as the research object, the difference in the three-dimensional fluorescence spectrum of soil dissolved organic carbon (DOC) after returning maize straw to the field at different soil depths (0~2, 3~10, 11~20, 21~30 and 31~40 cm) were analyzed. The change characteristics of humification degree of maize straw return to deep soil were discussed. The results showed that straw return could increase the content of soil DOC. The characteristics of three-dimensional fluorescence spectra showed that there were two kinds of fluorescence components of soil DOC. CK~T4 treatments were humus like components (Ex/Em=250~275/455 nm) and tryptophan like components (Ex/Em=225~237/340~350 nm), while T5 treatment was humus-like components (250~275/455 nm) and tyrosine like components (Ex/Em =225/304 nm), there were small authigenic components at a depth of 31~40 cm, and the humification coefficient was the highest. The fluorescence intensity of soil DOC component C1 increased with the deepening of straw returning depth, while the C2 component showed a fluctuating state, and the fluorescence intensity increased first and then decreased. Soil DOC was affected by both endogenous and exogenous sources (FI>1.4, 0.6