The invention relates to a calculation method for the impact height of viscous mud-rock flow on a slit damp. Mud-rock flow disaster preventing project prospecting is carried out on a mud-rock flow for engineering prevention and control, under the condition that the mud-rock flow density of the trench is greater than 2000kg/m3, and the flow velocity of the possible mud-rock flow, the mud depth, the trench width, the trench gradient, the opening width of the slit dam and the number of openings are determined; according to basic parameters obtained by field investigation, a froude number of theupstream mud-rock flow of the slit dam and the opening rate of the slit dam are calculated; and according to the froude number of the upstream mud-rock flow of the slit dam and the opening rate of theslit dam, the impact height of viscous mud-rock flow on the slit damp is calculated. According to the method, principles are reliable, the calculation process is scientific and simple, a calculationresult is highly precise, influence of the flow state of the mud-rock flow, geometric parameters of the slit dam and the trench condition on the impact process of the mud-rock flow is taken into integrated consideration, and basis is provided for the mud-rock flow disaster preventing project.