The invention discloses a preparation method of a completely biodegradable resin made of non-staple grain 
plant modified starch. The method comprises the steps of: first putting non-staple grain 
plant starch into a reaction vessel, adding water to prepare it into 
starch milk, and heating it to a proper temperature, conducting stirring and washing, then adding chemicals into a water solution for modification 
grafting treatment, and then separating water and 
starch through 
centrifugation, and conveying the separated 
modified starch to an 
airflow drying system for 
dewatering and 
drying, then taking 100 parts of dried 
modified starch, adding 5-50 parts of a compatilizer, 2-15 parts of an anti-block agent, 0-100 parts of 
polylactic acid, and 150-250 parts of a completely biodegradable 
copolymer of 
dibasic acid and dibasic 
alcohol, and mixing them uniformly, carrying out 
extrusion granulating or tabletting under an appropriate 
temperature and pressure, thus obtaining a resin material or sheet special for complete 
biodegradation. The resin of the invention has both the usage function similar to ordinary plastic products and a complete biodegradability. And after being used and abandoned, the resin can be degraded into 
carbon dioxide and water that can be consumed by the environment.