In order to better understand the variation characteristics of mixed oil segment in pipeline oil transportation, with the aid of the professional fluid simulation software Fluent, the numerical simulation is carried out for oil transportation shutdown pipeline under different angles, with simulation results and the derived data analyzed. The results show that during shutdown, gravity and the density difference between oil is the main factor resulting in changes of characteristics of mixed oil segment. As pipe inclination angle goes larger, natural convection generated by diesel in high gravitational potential energy increases the length of mixed oil segment significantly, and the time required is even shorter. When the gasoline is in high gravitational potential energy, the increase of pipe inclination angle inhibits the increase of the mixed oil segment and the concentration layering of mixed oil segment is consistent with pipeline inclination angle. The results of the study provide reference for the reduction and prevention of mixed oil segment generation and increasing under shutdown.