Debris flows are one of the common natural hazards in mountainous areas.They often cause devastating damage to the lives and property of local people.The sabo dam construction along a debris flow valley is considered to be a useful method for hazard mitigation.Previous work has concentrated on the different types of sabo dams such as close-type sabo dam,open-type sabo dam.However,little attention has been paid to the spillway structure of sabo dam.In the paper,a new type of spillway structure with lateral contraction was proposed.Debris flow patterns under four different spillway structures were investigated.The projection theory was employed to predict trajectory of debris flow out from the spillway and to estimate the incident angle and terminal velocity before it plunged into the scour hole behind the sabo dam.The results indicated that the estimated data were in good agreement with the experimental ones.The discrepancy between the estimated and experimental values of main parameters remained below 21.82%(relative error).Additionally,the effects of debris flow scales under different spillway structures were considered to study the scour law.Although the debris flow pattern and scour law behind the sabo dam under different operating conditions was analyzed in this paper,further study on the scour mechanism andthe maximum scour depth estimation based on scour theory is still required in the future. Debris flows are one of the common natural hazards in mountainous areas. They often cause devastating damage to the lives and property of local people. The sabo dam construction along a debris flow valley is considered to be a useful method for hazard mitigation. Previous work has has concentrated on the different types of sabo dams such as close-type sabo dam, open-type sabo dam. However, little attention has been paid to the spillway structure of sabo dam. in the paper, a new type of spillway structure with lateral contraction was proposed. Detection of debris patterns under four different spillways structures were investigated. The results indicated that the estimated data were in good agreement with the experimental ones. Discrepancy between the estimated and experimental values ​​of the main parameter s remained below 21.82% (relative error) .Additionally, the effects of debris flow scales under different spillway structures were considered to study the scour law .Although the debris flow pattern and scour law behind the sabo dam under different operating conditions was analyzed in this paper, further study on the scour mechanism andthe maximum scour depth estimation based on scour theory is still required in the future.