This work is focused on numerical simulation of the interaction of a solitary surface wave with a submerged rectangular breakwater in the constant depth model basin using the non-hydrostatic hydrodynamic model SWASH (Simulating WAves till SHore). The features of the transformation of a wave passing over an obstacle with the width and height changes of the breakwater were investigated. Based on numerical experiments, the transformation coefficients for the soliton and the zone of its attenuation area behind a breakwater were calculated. The localization of the region of maximum attenuation of the wave passing beyond the breakwater is determined. The analysis of the spatial structure features of free-surface fluctuations caused by the interaction of a soliton with a breakwater is carried out. The depthaveraged orbital fluid velocities are calculated and the dependence of their values and directions on the geometric parameters of the underwater obstacle is determined.