NIn order to reveal the effect of mixed exogenous carbon returning of crop straw and litter leaves on farmland soil quality in black locust-maize agroforestry system in Loess Hilly Region. In this study, based on a four-year positioning experiment, six treatments were set up, including 6 000 kg/hm² straw returning (Y4C0), 4 500 kg/hm²straw and 1 500 kg/hm² Robinia pseudoacacia leaf litter returning (Y3C1), 3 000 kg/hm² straw and 3 000 kg/hm² Robinia pseudoacacia leaf litter returning (Y2C2), 1 500 kg/hm² straw and 4 500 kg/hm² Robinia pseudoacacia leaf litter returning (Y1C3) and 6 000 kg/hm² Robinia pseudoacacia leaf litter returning (Y0C4), with no returning treatment as control (CK).The results showed that: 1)There were differences in soil physiochemical and biological properties among different soil layers. Compared with CK, Y3C1 and Y1C3 treatments significantly increased the contents of soil water, organic carbon, available phosphorus, nitrate nitrogen and ammonium nitrogen in 0-20 cm soil layer, meanwhile, the activities of β-1,4-glucosidase, N-acetyl-β-glucosaminidase and leucine aminotranspeptidase in this layer were also significantly increased by 55.30%-66.35%, 91.41%-101.26%, 24.58%-26.20%, respectively. But various treatments had little effect on soil properties in the 20-40 cm and 40-60 cm soil layers. 2)Y4C0, Y3C1, Y2C2, and Y1C3 treatments significantly increased soil quality index (SQI) compared to CK, and Y3C1 treatment had the best improvement effect, which was significantly increased by 14.44% compared with CK. 3)Random forest analysis showed that soil alkaline phosphatase activity, available potassium, nitrate nitrogen and available phosphorus contents were the key factors affecting the change of SQI. Comprehensive analysis showed that SQI and maize yield were the highest under Y3C1 treatment, which can be recommended as a suitable ratio of straw and litter returning to the field to improve the quality of farmland soil in the agroforestry system in the study area.