In order to explore the characteristics of runoff and sediment yield and the relationship between runoff and sediment yield on the slope under different antecedent soil moisture conditions, this paper took Ansai loessial soil as the research object, and systematically studied the influence of antecedent soil moisture on runoff and sediment on slope by setting six antecedent soil moisture treatments (5%, 10%, 15%, 20%, 25%, 30%). Two repetitions were set for each water content treatment, the slope gradient was 15°, the rainfall intensity were set to 90 mm/h, and the rainfall duration was set to 1 hour. The results showed that: (1) With the increase of antecedent soil moisture, the initial runoff time decreased as a power function, and the runoff increased linearly. (2) The amount of soil loss on the slope increased as a power function with the increase of antecedent soil moisture. The amount of soil loss in the high antecedent soil moisture group (29.3%) was 86.1 and 8.9 times that in the low water content group (5.8% and 10.6%) and the medium water content group (15.3%, 20.4% and 25.1%). When antecedent soil moisture was close to saturation, the slope erosion intensified and the amount of soil loss increased rapidly. (3) Under the conditions of different antecedent soil moisture, the relationship between slope runoff and sediment yield was a power function. When the runoff yield rate exceeded 1.4 L/min, the sediment yield increased rapidly or even doubled, showing the characteristics of "large water and more sediment". Antecedent soil moisture affected slope sediment yield by affecting infiltration runoff and changing the relationship between runoff and sediment. Therefore, for the loess slope, attention should be paid to preventing soil erosion caused by rainstorm or extreme rainstorm after continuous and lasting rainfall. Measures such as cultivation and vegetation cover can be taken to maintain appropriate soil water content and make the soil layer unsaturated to reduce erosion. The results provide a scientific basis for the protection and management of soil and water resources in the Loess Plateau.