Abstract: Abstract: There is a large straw production in the wheat-maize rotation system of Huang-Huai-Hai Plain in China, which increases the difficulty of straw utilization so that soil fertility cannot be effectively supplemented and the environmental pollution caused by straw burning is aggravated. In view of the difficulty of the large straw returning which is not conducive to rapidly improve soil fertility, the pelletized straw returning was created, which made the same area of farmland hold more straw. But it is unclear that if increasing the amount of pelletized straw has a positive effect on soil organic carbon (SOC) content. Therefore, the aim of this article was to explore the effects of the high and low straw return dosages on SOC content, SOC accumulation rate and soil nutrient elements ratio in 0-20, >20-40 and >40-60 cm soil layer and the yield of wheat-maize during the two-year trial period. Five kinds of straw managements were treated by 2-year field location test, including non-straw returning (CK), 12 000 kg/hm2 pelletized straw returning (KL1), 36 000 kg/hm2 pelletized straw returning (KL3), 12 000 kg/hm2 chopped straw returning (FS1), and 36 000 kg/hm2 chopped straw returning (FS3). Each straw treatment was a mixture of half wheat straw and half corn straw. The results showed that the SOCs in >20-40 and >40-60 cm soil layers were significantly affected by the 2-year straw returning. Among the four straw returning treatments, FS1 had the lowest increase, which was only 7.2% in >20-40 cm soil layer and 5.9% in >40-60 cm soil layer higher than that of CK, however, KL3 had the highest increase, which was 12.3% in >20-40 cm soil layer and 11.1% in >40-60 cm soil layer higher than that of CK. Compared with the chopped straw returning treatment, the pelletized straw returning treatment rapidly increased SOC content, which manifested that SOC content of KL3 was 1.7% in >20-40 cm soil layer and 1.3% in >40-60 cm soil layer higher than that of FS3 after 1-year straw returning, and SOC content of KL1 was 0.8% in >20-40 cm soil layer and 0.7% in >40-60 cm soil layer higher than that of FS1 after 1-year straw returning. In addition, high dosage straw returning had the advantage of improving SOC content greatly. Compared with the low dosage straw returning, FS3 increased SOC content by 1.7%-3.9% in >20-40 cm soil layer and 0.7%-3.8% in >40-60 cm soil layer, and KL3 increased SOC content by 2.4%-4.7% in >20-40 cm soil layer and 1.3%-5.1% in >40-60 cm soil layer. KL3 also had the higher SOC accumulation rate in each growing season, and the overall mean value was the highest among the five treatments. What's more, KL3 improved soil carbon-nitrogen ratio (RCN), carbon-phosphorus ratio (RCP) and carbon-potassium ratio (RCK) to a certain extent, which indicated that high dosage pelletized straw returning promoted soil nutrient conversion to a higher fertility direction. In this experiment, KL3 significantly increased the wheat yield by 4.57% and 10.87%, the maize yield by 11.40% and 8.87%, respectively in the 2-year trial period. In a conclusion, 36 000 kg/hm2 pelletized straw deep returning is the most beneficial to promote the SOC in the Huang- Huai-Hai Plain, which has a great significance in solving the problem of soil "carbon starvation" and ensuring the sustainable development of agriculture.