Abstract: The flow erosion intensity on soft mattress can be reduced effectively in the way of immersion along the flows. However, as the force condition is complex during the sinking process of soft mattress, it is difficult to ensure the structural safety. The force on the soft mattress in deep water and immersion along the flow conditions is analyzed. Field tests are carried out to validate the theoretical analysis results. Based on the force analysis, a new design method for soft mattress in deep water and immersion along the flow conditions is proposed. The structure has been well used in the middle and lower waterway regulation project of Yangtze River.