Based on in depth studies of structure of membranes, the principle of water osmosis in single direction was put forward, and water osmotic plastic film was designed and developed. Plastic film of the kind possesses various functions such as temperature preservation, temperature regulation, water percolation, water conservation and micro air ventilation.The tests covered with water osmotic plastic film showed an increase of 15 7%～38 3% in grain yield, 1%～4 5% of increase in soil water over conventional membrane, and double increase in grain yield than no covered.