Provided is a process for preparing a plant fiber composite wall plate, belonging to wall material field. The invention resolves the shortages that the raw material of the existing architectural wall material is large in consumption, the manufacture cost is high and the distribution of the raw material is non-uniform. The process steps of the invention are that crop straw is cleaned and cut into pieces and is ground by a fiber re-chipper, and then is dried after dust and big lump material are screened. The prepared raw material is sent into a glue blend machine and is added with adhesive agent of 10-16%. After being uniformly stirred, the raw material and the added adhesive agent are taken out, added with fire retarding agent and paved, and then are cut in standard size after being pre-pressed, hot-press-molded and cooled. The plate in standard size is combined with polystyrene plates via different layers and thicknesses, the surfaces thereof are stuck with melamine fireproof plates, and then the plant fiber composite wall plate is formed via the combination. The product obtained by the process has the advantages of low density, light weight, perfect insulation, simple technology, large raw material source, low manufacture cost, waste utilization and environment protection. The invention can be used to produce building separating walls, framed wood structural buildings, mobile movable dwellings, and buildings in tourist spots and the like.