Extractable polycyclic aromatic hydrocarbons (EPAHs) and bound PAHs (BPAHs) were measured in a sediment core using conventional Soxhlet extraction and a more astringent extraction method, with the objectives of determining the influence of BPAHs on the historical reconstruction of PAHs and exploring the formation of BPAHs and long-term behaviors of PAHs in sediment. The results indicated that the formation of BPAHs was clearly sediment-depth and molecular-size dependent. BPAHs represents an important portion of PAHs in sediment and cannot be extracted by conventional Soxhlet extraction. This suggests that the previously developed vertical profile of PAHs is not the real chronology of PAHs and the plausible interpretation derived from the sedimentary records of PAHs needs reexamination. Based on the previous findings, a biphase model was proposed and the formation of BPAHs was predicted. Due to the different nature of geosorbents in sediment, redistribution of PAHs among these geosorbents logically leads to the formation of BPAHs and is kinetically favorable for smaller molecular PAHs. This is consistent with the obtained results. Many factors may influence the formation of BPAHs, such as the physicochemical structure of sediment and environmental conditions. There is still a long way to reveal the thermodynamical characteristics in action during the formation of BPAHs.