Soil erosion on sloping farmland has posed a great challenge on land productivity and crop yield, resulting coarse soil and less arable area due to the losing of fine soil particles. Moreover, N and P in soil that migrated with runoff and sediment can also affect the quality of downstream water. Therefore, it is highly demanding to control soil erosion on sloping farmland. Currently, various measures are being taken in the long run, covering from biological, tillage, and engineering. Soil amendments can be used to greatly reduce soil erosion and increase cohesion between surface soil particles, which can maintain surface soil structure to prevent soil crust, resulting the increase in soil infiltration rate, while the decrease in the surface runoff. Therefore, a new material related to soil amendments has drawn much attention. In recent years, the Changjiang River Sciences Research Institute has developed a new seaweed polysaccharide Anti-erosion Material SA-01. This study aims to explore the effect of SA-01 concentration on runoff and sediment yield of red soil, in order to verify the new polysaccharide corrosion inhibitor (SA-01) from the seaweed in controlling soil and water loss on a slope. Taking the typical red soil in southern China as an example, a series of artificial rainfall experiments were carried out, where the rainfall intensity of 90 mm/h, and the slope of 5°, 10°, and 15° under various concentrations of 0, 0.25%, 0.50%, 0.75%, and 1.00%. The mechanism of SA-01 on soil erosion was analyzed in a combination of soil water repellency and aggregate stability experiments. The results show that under the same slope, the runoff yield on the slope with SA-01 was higher than that on the control slope. The runoff yield on the slope decreased first and then increased, while the cumulative runoff on the slope decreased first and then increased with the increase of applied concentration. The increment ratio of surface flow increased as the SA-01 concentration increased, indicating that SA-01 can significantly reduce sediment yield on a slope. In the initial runoff yield stage, SA-01 can significantly reduce the slope sediment yield. In the stage of surface erosion, the slope without SA-01 reached the maximum, then fell rapidly, and eventually became stable. Compared with the control slope, the sediment yield of SA-01 slope fluctuated slightly in the early stage of runoff yield, and the time to reach stable sediment yield was earlier than that of the control slope. With the increase of applied concentration, the sediment yield decreased significantly. SA-01 can significantly increase the runoff while reduce the sediment yield of different slopes, due mainly to the change of soil hydrophilic to water repellency, and the decrease of infiltration rate. Soil water repellency was improved after SA-01 was applied. When the concentration was 0.25%, the sample retained the same hydrophilicity as the original soil. When the concentration was more than 0.25%, the soil began to change from hydrophilicity to repulsion. Due to the soil Ca2+ cation chelation with SA-01, the soil particles generated on the surface of a layer with a certain intensity. The presence of preservation layer increased soil water repellency, while reduced the soil aggregate dispersion, and thereby to improve the stability of soil aggregates at different levels. SA-01 can provide a new idea for soil and water conservation of red soil on a slope.