Abstract: Based on the study of structures and oil sources in the F117 Well Area of the Fulin Sag made by the previous researchers, downward petroleum migration in the Mesozoic of this area was proposed for the first time in this paper. Through recovery of the pressure using fluid inclusions, combined with the observation of rock thin sections and core, the downward migration of oil and gas is proved from two aspects respectively, i.e. the overpressure and crack passage system. Using mathematical deduction as the method, introducing the concept of "pressure drop", the maximum downwards transportation of oil and gas may reach the range of 172.84m, fit with the figure of 180 m from oil production test.