High sequestration efficiency and strong sequestration potential of land-sea interaction regions are important nature-based long-term solutions to climate change and the achievement of “carbon peak” and “carbon neutrality”. To reveal the sources and forms of Dissolved Organic Matter (DOM) in the land-sea interaction stratigraphy along the south coast of Laizhou Bay and enrich the understanding of carbon sources and sinks in the land-sea interaction region, interstitial water from a drill core was collected from November to December 2021 and analyzed for ultraviolet (UV-) visible absorption spectra and three-dimensional fluorescence spectral signatures of Chromophoric Dissolved Organic Matter (CDOM). The analyses showed that dissolved organic carbon and a(320) were both characterized by large and high concentrations after the second transgression. The differences in the DOM composition and properties of terrestrial-phase interstitial water were greater than those of marine-phase interstitial water. Excluding the bottom stratum, of the five fluorescence components identified by parallel factor analysis and matched using OpenFlour, humus-like components contributed 77% of the total fluorescence intensity. The fluorescence and biogenic indices showed that the core interstitial water chromophoric dissolved organic matter was dominated by the contribution of microbial reproduction activities; the longer the burial was, the more significant the microbial contribution, and there was no significant difference between the terrestrial and marine phases. The results of principal component analysis show that the compositions exhibit several differences in different sedimentary stages, but the composition and properties of the interstitial water DOM gradually converge as the sedimentary process occurs. Humus matures over long timescales.