Abstract: Geotextile mattress with sloping plate (GMSP), a new countermeasure for scouring control, was introduced for scouring protection below underwater pipeline. A flow-seepage coupling model was established for the simulation of flow field and seepage field adjacent to the pipeline in a steady current. The protection effect of GMSP on a pipeline was validated. The influence of GMSP design parameters on the protection effect was also investigated. The results showed that GMSP is capable of decreasing the flow velocity near the pipeline and the seepage gradient below it, thus protecting the pipeline from the scouring. The seepage gradient reduces as the sloping angle of the plate and the plate height increase, and increases as the sand-pass opening increases. The result indicated a negative non-linear correlation between the seepage gradient and the effective plate height. When the effective plate height is over a critical value, the seepage flow under the pipeline reverses, i.e. in the opposite direction of the mainstream.