Long distance transmission pipeline is one of promising transportation methods developed in recent years. It is safe, economical, convenient and prompt. It is mainly used in transmitting gas, liquid, and other dispersed material. Oil and natural gas is especially suitable to be transmitted by high pressure large diameter longitudinal submerged arc welded (LSAW) pipelines. Mechanical expanding is one of the most important processes in the production of large diameter LSAW steel pipe for long distance transmission pipeline. Firstly, the LSAW steel pipe mechanical expanding is modeled. Two FEM models are established to simulate the mechanical expanding process at the cross section and longitudinal section of the LSAW steel pipe respectively. Secondly, the deformation characteristics of the LSAW steel pipe are simulated while mechanical expanding processes. Finally, main mechanical expanding process parameters and their influence on the quality of finished LSAW pipe are analyzed in detail according to the FEM simulation. The results presented by the analyses are very consistent with the experiment, and can be used to direct the production of LSAW steel pipe.