Gravity is the most fundamental biomechanical stimulus for posture change. Pressure of blood flow is one of primary indicators to evaluate cardiovascular performance. Up to now, the underlying mechanism of effects of posture change on cardiovascular system is still unclear. A 3D FSI mathematical model with posture change was presented. By applying the body force terms to the fluid equation and the vessel wall equation, the model could be used to study posture change and the effects of gravity on the pressure of blood flow quantitively. Under different inlet-outlet pressure difference(IODP) and different postures such as horizontal, upright and upside-down one, the effects of gravity were simulated. In horizontal position, the pressure distributions of blood flow transformed from 2D(two-dimensional) axis-symmetry without gravity to 3D asymmetry with gravity under small IODP. With IODP increasing, gravity had less effects on pressure distribution and extreme value. As IODP reached 10 665.6 Pa(80 mmHg) and 2 666.4 Pa(20 mmHg) respectively, this effect was observed to be constant. Similar results were obtained from 3D fluid-only model. In either upright or upside-down position, 2D axis-symmetric pressure distribution was observed with and without gravity, yet the position, in which extreme pressure appeared, was different in upright position from that in upside-down one. Finally, the effect intensity of gravity in upright or upside-down position was more than twice as much as that in horizontal one. The results indicate that introducing body force term into the fluid and solid equations to present a novel model, which was based on hemodynamics, will provide a new way to study posture change. Effects of gravity on pressure distribution and extreme value changed with different postures and IODP. If IODP is small, ignoring effect of gravity and postures so as to simplify the hemodynamics model to 2D axis-symmetric one, the conclusion should be drawn with caution.