In order to study the effects of the seabuckthorn flexible dam on sediment in the soft sandstone gully, field experiments were carried out in the soft rock region of Inner Mongolia, and based on the experimental data of single seabuckthorn flexible dam after a rainstorm, effects of seabuckthorn flexible dam on sediment particle size distribution and organic matter and sediment retention effect of the dam were analyzed through employing the method of comparison. Results showed that single seabuckthorn flexible dam improved the organic matter content of gully sediment, and it had silt separation and fine sorting function and good sediment retention effect. The content of sediment organic matter increased from upstream to downstream of the seabuckthorn flexible dam, and the content of organic matter in middle sediment layer was higher than those in surface and bottom layer. The sediment particle size increased first and then decreased from upstream to downstream of the gully, and the same tendency appeared along the sediment profile of a rainfall deposit. After a rainstorm, the average thickness of silt deposited in unit area of seabuckthorn flexible dam was 0.369 m, and it was significantly higher than that of the control group.