The invention discloses a rib threshold design method of a soft-foundation energy-dissipation debris flow drainage groove. The drainage groove comprises a plurality of rib thresholds arranged according to a certain distance, and side walls on two sides; the rib thresholds are in arched shapes along the negative direction of a debris flow movement direction; and arch supports are inserted into foundations of the side walls or are connected with the foundations of the side walls into a whole. The arched-shaped rib threshold design method comprises the steps of firstly determining the semi-arch width of each arched-shaped rib threshold according to the designed width of the drainage groove; then determining a load characteristic coefficient according to a soil pressure load and a debris flowimpact load borne by each rib threshold; then determining the characteristic length of a side support according to the maximum horizontal support force of each side wall foundation on the corresponding arch support; and finally determining an optimal arch axis equation. According to the method, each rib threshold is designed to be in the arched shape, and meanwhile, the optimal arch axis equationof each arched-shaped rib threshold is designed, so that the problem that the rib thresholds are damaged due to the failure of a soil boy support after the rib thresholds and the increased load beforethe rib thresholds can be effectively solved, and a simple, convenient and effective rib threshold optimization method is provided for areas and working conditions with higher damage rate of traditional straight rib thresholds.