The scroll extrusion forming is precision plastic processing which is close to the die strength limit，and the die working environment is very bad which requires higher requirement for the precision,strength,stiffness and servicelife of the die. According to the shape of the scroll, the die structure using back pressure principle was designed and the force condition was improved by adding outer mold set in die outer. The Deform was used to simulate stress and displacement of combined die.The simulated results show that the die internal stress is reduced obviously with the increase of the outer mold set and each displacement is also reduced, which improves the die strength and service life.