Analysis of long span cable bridge such as: Cable stead or Suspension bridges under the effect of fluid structure interaction (FSI) became the main parameter affect design and safety of long spans bridges. In this research we will discussed, the 3D-CFD Finite element modeling of the wind flows around bridges. The 2D-modeling of the bridge deck and the fluid domain around it to get the critical wind speed without flutter occurs, aren't more accurse in the bridge modeling. There are many out of plane parameters that affect the critical wind speed and flutter. But also the 3D-Modeling of the bridge and the fluid domain using the finite element theory in all of computer programs are very difficult because it need a huge disk space and long time to run the analysis. Therefore the research studies the reshape of fluid interaction 3D-domain, and How to make it optimum in analysis, time, and efficient results. The optimum 3D-domain takes every in plane and out of plane parameters the affect the long span bridge modeling. That gets accurate nearest simulation to bridge behavior in fluid structure interaction closed to the wind tunnel test.