The paper is focused on the description and creating of the simulation model of the power assisted steering and testing of the functionality and efficiency on the laboratory test rig. The mathematical model is describing the structure of the system and is created from the mathematical models of the subsystems, which compute the dynamic behaviour of the steering wheel, pinion, rotating control valve and of the hydraulic cylinder. The simulation model was realized in the simulation programme MATLAB-Simulink® and was used for the simulation of the testing experiment. The measurements were done on the laboratory test rig, which is controlled by the computer and operated by the swivel pneumatic drive instead of the driver hand. It allows better repeatability of the experiment. The experiments were focused on the evaluation of the efficiency of the hydraulic power assisted steering system.