Abstract: As a key microtopographic feature, debris flow constrictions are of vital importance in the design of debris flow prevention and control projects. By analyzing the genesis, types, scouring-deposition patterns, and effects on the blockage-breach processes of debris flow constrictions in Cutou Gully, this paper aims to provide theoretical support for debris flow disaster prevention and mitigation decision-making. Taking Cutou Gully in the Minjiang River Basin as the research area, field investigation and correlation analysis were adopted to identify the formation conditions and scouring-deposition characteristics of debris flow constrictions, and to reveal the control of material composition and geometric morphology on the blockage-breach process. The formation of constrictions is mainly controlled by geological structure, lithology, and sediment accumulation in the constriction segments. Constrictions can be systematically classified according to material composition, planform morphology, and cross-sectional shape. Their scouring-deposition characteristics can be summarized into three types: deposition-scouring-deposition, deposition-scouring-scouring, and scouring-deposition-deposition. Deposit-type and composite-type constrictions are prone to blockage, while bedrock-type constrictions mostly cause discharge attenuation. An increase in the cross-sectional area ratio between the upstream and downstream sections of a constriction promotes discharge amplification, whereas increases in longitudinal slope ratio, constriction ratio, and widening ratio tend to suppress discharge amplification. The classification system of debris flow constrictions and its response laws of movement and deposition established in this paper can provide a theoretical basis for risk assessment of constriction sections and decision-making for debris flow disaster prevention and mitigation engineering in the Cutou Gully area.