Facility soils have a long-term problem of insufficient carbon input，and straw pellets are one of the important ways to achieve significant input of exogenous carbon.In this study，two types of straw(crushed straw and straw pellets)and four levels of straw return(1%，2%，3% and 4% of dry soil weight)were used in a pot experiment，and no straw return was used as a control(CK)to study the effects of different amounts of straw pellets on the enzyme activities related to carbon and nitrogen cycle in the facility soil.The results showed that:(1)Both straw pellets and crushed straw returned to the field Showed positive effects on the activities of cellulase，β-glucosidase，α-glucosidase，urease，leucine aminopeptidase，acetylamino glucosidase，glutamine synthetase，acid phosphatase，but Showed negative effects on phenol oxidase activity.The activity of catalase showed a positive effect when a small amount of straw was returned to the field and a negative effect when a high amount of straw was returned to the field.(2)The soil enzyme activities of both straw pellet and crushed straw treatments changed with the increase of the amount returned to the field first increase and then decrease，and the difference between straw pellet and crushed straw treatments was mainly related to the level of the amount returned to the field.The effect on the enzyme activity was reduced by the positive effect after the amount of return to the field exceeded 2% and reached a certain level.The straw pellets were more favorable to improve the enzyme activity related to soil carbon and nitrogen cycle under the high volume return condition above 3% return level.(3)The straw pellets significantly increased the soil nutrient content in large quantities.(4)The soil nutrient content was significantly related to the soil enzyme activity and was the main control factor for the change of soil enzyme activity.(5)The suitable return amount of straw pellets was 3%-4%，while the suitable return amount of crushed straw should not exceed 2%.In conclusion，straw pellets at a high return level of 3%-4% can significantly improve soil nutrient content and soil enzyme activity，and is an effective way to improve the high amount of carbon fertilizer input to the soil of carbon and nitrogen imbalance facilities.