A heat pressing method for polyethylene plant fiber composite material relates to a method for manufacturing plant fiber material. The invention solves the problems that only fibril can be applied, plant format is narrower, straw is liable to thermolysis and charring and the mechanical properties of plant manufactured by pre-heat pressing are poor in the production of WPC in the manner of extraction. The method of the invention has the steps that: PE plastic fiber is mixed with crushed plant aggregates to obtain mixture that is paved into a plate base; the plate base is pre heated under a pressure of 1 to 3 MPa and at a temperature of 170 to 190 CEG D; the plate base is kept under a pressure of 4 to 6 MPa for 8 to 15mins; and at last, the plate base is cooled to 70 to 80 CEG D under a pressure of 3 to 6 MPa; and after pressure relief, the plank is obtained. The flexural strength of the plank manufactured in the method of the invention is 20 MPa. The invention adopts the polyethylene in fiber configuration and the crushed aggregates of the plant and the fiber is evenly mixed and has high productivity. In addition, the crushed aggregates of the plant and the fiber can be prevented from being deteriorated and darkened due to high temperature.