Improving the knowledge about the influence factors of the allowable free-span length which required in pipeline design, this study provides information about an evaluation of fatigue life due to vortex induced vibration (VIV). Standard finite element techniques are used to the eigen value analyses of a oil-conveying free-span on seafloor. Combined with VIV response model, the fatigue life under three different end boundary conditions (fixed, pinned and elastic seabed) is evaluated. The influence parameter study has shown that the allowable span length is very sensitive for structural natural vibration characteristic, controlled by axial force, end conditions and internal flow. The natural frequencies of free span increase while both the internal flow speed and compression decrease, and internal flow velocity was found less important for VIV response than other functional load factors, because the speed in reality is not high enough.