Numerical simulation was made on hot backward extrusion of large steel tube with thick wall and the influence of friction coefficient and wall thickness on flow state of metal was analyzed. The result shows that when the inner part of the cylindrical metal shifts upwards, it also flows along the radial direction. The larger the friction coefficient and the wall thickness, the more metal flows horizontally, and the punch is more likely to be sticked. According to the simulation, a new forming area partition method was proposed about the large thick wall tube on hot backward extrusion, which adds radial plastic deformation area on existing forming area partition method of backward extrusion. Research proves that this method is in accordance with metal flow condition well.