[Objective] The objective of the study is to investigate the effect of the seabuckthorn plant flexible dam (SPFD) on the longitudinal distribution of flow velocity and water depth, and to analyze the variation of the roughness coefficient and related influencing factors, thus providing the theoretical support for the application and extension of the SPFD, one of the ecological engineering for soil and water conservation, in the mountainous region. [Methods] We carried out the field flow experiment and used the theoretical analysis to investigate the effect of the SPFD on water flow and the variation of gully roughness coefficient. We also assessed the effect of the SPFD on the longitudinal distribution of flow velocity and water depth, and analyzed the roughness coefficient variation along the pathway direction under the different slopes of the test bed. [Results] The amplitude of flow velocity variation was related to the dam length, the plantation density of the seabuckthorn and the test bed slopes. The flow velocity variation amplitude increased with the increasing slope of the test bed with the longer SPFD. The heap water depth and the roughness coefficient at the upstream zone increased with increasing slope within the test bed with the higher seabuckthorn plant density. [Conclusion] The seabuckthorn plant flexible dam has obvious blocking effect on flow velocity in the front of the dam, and the dam length and the plant density are the main factors affecting the bloocking effect of the SPFD.