This paper presents a methodology for describing the relationship between forces acting on the cutting head of an abrasive waterjet machine and the structural response of its supporting structure. The study was conducted in two stages. First, experimental measurements of reaction forces during high-pressure AWJ cutting were performed to determine the influence of process parameters and define the loading conditions. Next, a finite element (FEM) model of the supporting structure was developed, in which the cutting head was idealized as a reference point to enable controlled loading in three directions. A series of simulations with unit loads was carried out to determine the compliance matrix of the structure. The model allows rapid prediction of sensor displacements for arbitrary force combinations without repeated FEM analyses. Experimental validation showed good agreement, with a relative error of approximately 8% in the direction of the lowest stiffness, demonstrating the applicability of the proposed approach for monitoring loads in AWJ processes.