On the basis of thermo-mechanical coupled FEM model which characterized the composites liquid-solid extrusion process, the constant-velocity and non-constant-velocity extrusion process were simulated by 3D thermo-mechanical FEM. Their influence on forming temperature, forming quality and deforming force was analyzed and compared. In the constant-velocity extrusion process, the results show the over-high and low forming temperature in earlier and terminal extrusion stage were responsible for the surface annular cracks and inner fibers breakage. In the new non-constant-velocity extrusion process, however, the range of forming temperature can be shortened in a reasonable range and the over-high and low forming temperature were thus eliminated. Therefore the forming quality and its consistency can be improved. In addition, the deforming force can also be reduced. The simulation and the mechanisms analysis coincide with the experiment results reported in previous literatures. The study provides the theoretical gist and a new feasible technical scheme to favor the consistency of the extruded product quality in composites liquid-solid extrusion process.