Quantification of impact forces from submarine landslides is vital to the safe design of mudmats and other subsea structures that are laid out in the hazard area. In this paper, the dynamic process of the submarine landslides running across a partially-embedded mudmat is analysed using a novel numerical method, the material point method (MPM). The mudmat is simplified as fixed in position, presenting an exposure height and length above the mudline. The seabed is taken as a rigid boundary to exclude the complicated seabed-slide interaction. Increase of the undrained shear strength of the sliding material with its shear strain rate is expressed with the Herschel-Bulkley rheological model. Non-dimensional parameters, the non-Newtonian Reynolds number and the Froude number, are adopted to characterise the kinematic behaviour of the slide. The horizontal steady forces on the mudmat obtained from the MPM analyses are verified by comparison with those using a computational fluid dynamics (CFD) method. An extensive parametric study is then conducted, allowing the forces to be interpreted using a hybrid model considering the combined effects of the inertia of the slide, its shear strength and also the asymmetric static pressure.