The invention provides a method for preparing a plastic formwork by means of one-step forming. According to the method, a twin-screw extruder is adopted; by improving the twin-screw extruder and auxiliaries, recovered materials are conveyed to an extruder feeding inlet through a main hopper, the auxiliaries for processing and modifying are well blended and conveyed to the extruder feeding inlet through auxiliary hoppers, and the main hopper and the auxiliary hoppers control the proportion of the recovered materials and the auxiliaries which are added through a variable-frequency and variable-speed motor; and afterwards, a melt is propelled to a screen exchanger through extruder screws and then is connected into a die through a melt pump, and then the melt passes through a vacuum cooling shaping table, six sets of gear-type traction devices, a cooling bracket, two sets of gear-type traction devices, a ripsaw, a crosscut saw platform and a conveying belt, so that the process for preparing the plastic formwork by means of one-step forming is completed.