Abstract: Abstract: Straw residues left in the field include the crop stubbles, stems and leaves, part of the crushed straw remained in the field under the condition of reasonable harvest stubble. Besides collectable straw returned to field directly, leaving the straw residues in the field is the basic form of direct-application of straw into fields and this phenomenon could be widely seen throughout almost all the cropland in China. Researches on quantitative estimation of straw residues are of great significance to estimate the amount of collectable straw resources, evaluate the land productivity quantitatively and to make planning of direct-application of straw into fields etc. Formulas for estimating the quantity of straw residues left on the field were derived in this research. Five parameters were used in this study including average plant height, harvesting stubble height, leaf ratio, leaf fall rate, crops stalk loss ratio of mechanical collector. Parameters that estimated the quantity of wheat, corn and rice straw residues left in the field were determined by field surveys and literature analysis through selecting a large number of experimental data and survey data. The results showed that the amount of straw residues left on the field would increase gradually with the growing mechanical harvesting area. Total amount of straw returning to the field of China would increase to 600 000 000 t in ten years and that was 60% of the total production of crop straw. If the amount of collectable straw returning to field increased at the same time, the proportion and level of straw returning to the field would reach the level comparable to developed countries such as USA and Japan etc. In 2013, the amount of straw residues left on the field of three major crops of wheat, corn and rice in China was 71 069 200, 45 434 800 and 63 929 500 t, respectively. Straw residues left on the field of the whole nation was 253 300 800 t in the cropped area, and straw residues left on the field for the average per hectare of the sown area was 1.87 t. To meet the minimum requirement of the basic amount of straw residues directly returning to the field, this number should increase by 60% at least. Meanwhile, it should be expanded by 140% at least to meet the regular amount of straw residues directly returning to the field. The final part of this paper showed future needs for such research to calculate the quantity the straw residues applied into the field. The first is to carry out the extensive measure and calculation in the field, thus promotes the accuracy of the quantitative estimation parameters of straw residues of wheat, corn and rice, especially that of the harvesting stubble height and crops straw loss ratio of mechanical collector. The second is to carry out the quantitative estimation research on the straw residues of the economic crops as cotton, bean and rape etc., and further establish parameters of quantitative estimation of national major grain and economic crops' straw residues directly applied into the field. The third is to establish the parameters of quantitative estimation of regional straw residues directly applied into the field, and then gradually extend to apply this method to all the rest regions.