Pyrobitumen, considered a carbon-rich solid product of oil thermal cracking, widely exists in oil/gas source rocks and reservoirs. The importance of the polar fractions on the formation of pyrobitumen has been acknowledged. However, the precise role of the polar fractions in the formation of pyrobitumen is still not well understood. Since the heteroatoms of nitrogen (N), oxygen (O) and sulfur (S) mainly exist in the polar fractions, the investigation of their content and evolution characteristics during the formation of pyrobitumen will provide an understanding of the formation mechanism of pyrobitumen. In this study, laboratory pyrolysis experiments on three types of oil (normal oil, light oil and heavy oil) were conducted. The major pyrolysis products, such as gaseous compounds, liquid fractions and pyrobitumen, were quantitatively analyzed. The elemental compositions of carbon (C), hydrogen (H), O, N and S in the obtained liquid products and pyrobitumen were measured. From the yields of the pyrolysis products and the determined elemental compositions, we propose a three-stage formation scheme for pyrobitumen. In the first stage, the reactions are dominated by a cross-linkage of the polar fractions in crude oil that occurs at an Easy Ro>1.0% (H/C for pyrobitumen < ca. 0.8–1.0). In the second stage, the cross-linking reactions will occur with both the hydrocarbons (saturated and aromatic fractions) and polar fractions with an Easy Ro > 1.4–1.6% (H/C for pyrobitumen < ca. 0.6–0.7). In the third stage, almost all removable components in the crude oil are transformed into solid bitumen and CH4 at an Easy Ro>3.0% (H/C for pyrobitumen < ca. 0.4). This scheme is expected to be applied in studies to distinguish the original types of oil for pyrobitumen and establish a new thermal maturity index based on pyrobitumen and oil/gas resource assessment.