The oil source and accumulation process of Guantao Formation heavy oil in the Linfanjia area of the Jiyang Depression, Bohai Bay Basin, remain unclear. In this study, the source, genesis, and accumulation history of the heavy oil were investigated using biomarker and carbon isotope characteristics of crude oil, combined with fluid inclusion analysis, through reservoir and molecular geochemistry methods. The results indicate that the heavy oil in the Guantao Formation originated from mature source rocks of the upper sub-member of the fourth member of the Shahejie Formation in the Lijin sag. Biodegradation is identified as the primary mechanism responsible for oil thickening, with the estimated time required to reach the current level of degradation ranging from 4.4 to 13.0 Ma. The formation of the heavy oil involved two charging events and one biodegradation phase. The first was a small-scale oil charge during the deposition of the Paleogene Dongying Formation (31.2-24.6 Ma), followed by a large-scale oil charge during the Neogene Guantao to Minghuazhen Formation (15.0-7.0 Ma). Oil migrated to the Guantao Formation reservoirs in the Linfanjia area mainly via the Shangxi fault and associated sand bodies. Approximately 2 million years after accumulation (around 13.0 Ma), biodegradation began and continued, gradually increasing the oil 's viscosity and forming heavy oil. The accumulation process can be summarized as:initial charging, thickening during continued charging, and then sustained biodegradation-driven thickening. These findings provide theoretical insights into the genetic mechanisms, thickening processes, and associated shallow gas occurrences of heavy oil in the Jiyang Depression and other similar settings.