To improve the precision and to further complement and perfect the calculation theories for jet trajectory distance, a calculation method for the upper and lower jet trajectory distances in the perspective of the takeoff velocity was proposed. Based on the concentric circle assumption of the streamline on the flip bucket with the consideration of the centrifugal force and the hydrostatic pressure, the potential flow velocity distribution function was derived. Theoretical calculation formulas for the velocity of the upper and lower trajectory were obtained from the takeoff velocity distribution and then the jet trajectory distance calculation was realized. The calculation method was verified by both model tests and prototype observation datum, showing a reasonable accuracy for the upper and lower trajectory distance when the relative takeoff water depth is between 0. 15 and 0. 25.