An indoor simulation test system with a mud (sediment) water mass ratio of about 1:1 and a period of 56 days was adopted. Six concentration gradient test groups, including the control group, were set up to study the change characteristics of total organic carbon (TOC) content in sediments caused by different levels of exogenous phosphorus input. At the same time, UV-Vis absorption spectroscopy was used to study the effect of phosphorus containing substances on the structural changes of dissolved organic matter in sediments. The experimental results showed that the TOC content in the sediment of each experimental group decreased by 3.60%, 7.08%, 8.09%, 9.33% and 11.80% respectively compared with the control group on the 56th day, indicating that the decomposition of TOC in the sediment of the Yangtze Estuary accelerated with the increase of exogenous phosphorus input. During the test period, the maximum humification degree and aromatization degree of dissolved organic matter in sediments were positively correlated with the input of phosphorus. The molecular weight and aromatic ring structure of dissolved organic matter were positively correlated with the input of exogenous phosphorus, indicating that the increase of exogenous phosphorus input could accelerate the change of dissolved organic matter in sediments. With the increase of test time, the humification degree and aromatization degree of dissolved organic matter with different phosphorus levels increased first, and then began to decline; The molecular weight of dissolved organic matter gradually decreased with the increase of time, and the substituents on its aromatic ring substituents were gradually replaced by more active functional groups. It shows that humus is formed during the reduction of organic carbon content in sediments. As humus continues to decompose, the degree of humification decreases, and the degree of humification is the same as that of aromatization.