In this study, to make gas-assisted extrusion (GAE) can be better applied to conventional horizontal extrusion forming process, the thickness difference between up and down gas layer was simulated for the first time by GAE of molten high-density polyethylene sheet. In order to ascertain the effects of the thickness difference of up and down gas layers on sheet extrusion molding, a method based on the model of two-phase (gas and melt) flow was put forward, and an outer-layer GAE isothermal numerical simulation made by POLYFLOW was explored and researched. The numerical results analysis indicated that when the thickness of down gas layer is increased, the velocities, pressure drop and shear rate of the melt increase. Compared with the simplified-GAE simulation, GAE simulation method can well reflect the flow characteristics, the physical field distribution, and the morphology changes of melt extrudates. The experimental results show that with the increase of the thickness of the down gas layer, the melt falling can be obviously improved during extrusion. Thus, in the actual use of GAE for sheet production, not only the thickness difference between up and down gas layers should be reasonably controlled, but also other factors, such as melt inlet volume flow rate, gas pressure and temperature should be controlled accordingly. Finally, GAE forming technology can give full play to its advantages.