Mathematical model of fast hydraulic actuator dynamics, based on Riemann’s equations, is presented in the paper. Fast hydraulic actuator can be assumed as two serially connected compressible fluid flows controlled by supply and return variable flow restrictors enclosed in control servo-valve and separated by actuator piston. Exhibited dynamic model includes several physical effects, such as: fluid viscosity and compressibility, compression and expansion, wave propagation, actuator equivalent inertia, potential external load and arbitrary servo-valve control input. Solution of the presented mathematical model is evaluated by using the method of characteristics. Corresponding boundary conditions assume the actuator piston position as a movable boundary between the direct and the reverse part of the actuator chamber.