As the main source rocks in East China Sea Basin, the Paleogene source rocks of K area shows a general inversion phenomenon, which has not been paid attention to and explained at present. In this paper, the stratification and zoning of carbon isotopes in the source rocks of K area are studied by means of carbon isotope analysis, and the causes of inversion are summarized. The geological significance of carbon isotope inversion is described. The results show that the distribution of carbon isotopes in the source rocks in K area is dominated by local inversion. From Baoshi Formation to the upper section of Pinghu Formation, the distribution of carbon isotopes shows the characteristics of global normal → aromatic inversion → non-hydrocarbon inversion → saturated hydrocarbon and aromatic hydrocarbon double inversion. The inversion distribution of carbon isotopes indicated that this area was generally located in a transitional environment between sea and land, and the organic matter came from the mixed contribution of higher plants and lower aquatic organisms strongly degraded by bacteria. From the slope high belt to the western subdepression, the contribution of higher plants in sedimentary organic matter gradually decreased, while that of aquatic organisms gradually increased. The results of this paper have certain reference significance for the research of organic matter sources and sedimentary environment in adjacent or similar areas.