Based on the theory of plant micro-ecology, organic material decomposing agent（OMDA） was made by selecting beneficial bacillus, Trichoderma and yeast which can decompose straw quickly, and using modern microbial fermentation technology. Using OMDA to corn straw in the field could improve the content of soil organic matter, nitrogen, phosphorus and potassium by 19.28%, 5.95%, 7.35% and 1.77%, respectively. Crop production increased by 7.72%. Corn grain crude protein, crude starch and crude fat content properties also increased by 9.21%, 1.12%and 3.73%, respectively. Applying OMDA for rice straw in the field, yield increased by 20.15%, content of organic matter, nitrogen, phosphorus and potassium increased by 3.45%, 5.47%, 7.28% and 13.94%, respectively. Therefore, organic material decomposing agent promoted the effects of corn and rice straw returning the field significantly in northeastern China.