The flammable thermoplastics are widely used in our daily life and manned space travels in microgravity, posing a potential fire risk. This work studies the flame spread along with the thin polyethylene (PE) film (15-100 mu m) in the microgravity drop tower and normal gravity. Microgravity flame spread faster than vertically downward flame spread in normal gravity due to the weak buoyancy flow and greater flame preheating length. For the ultra-thin film, the influence of melting on the opposed flame spread is negligible. The upward flame spread rate reaches a maximum constant (45 +/- 10 mm/s for 20 mu m film) when the inclination angle is larger than 30 degrees, due to the dripping removal of molten fuel. The upward flame spread rate changes under the competition between the enhanced flame heating by buoyancy and the dripping removal of the molten fuel. The vertically upward spreading flame cannot be maintained due to the dripping removal of the molten fuel, and a critical extinction condition was determined and analyzed. This work provides valuable data on flame dynamics in plastic films and can help develop more sophisticated material flammability tests for fire safety in space travel.