The 2D digital servo valve studied here is a two-stage valve designed by using both rotary (angular) and linear motions of a spool. The rotary motion is driven by a stepper motor operating under continual angular displacement control, while the linear motion of the spool is actuated by hydraulic servo control with feedback of the spool's displacement, which is achieved by a unique “servo screw”.The modelling of the 2D valve is based on linear theory and is further verified by the special experiments. Because of the extremely large hydraulic natural frequency, the control of the 2D valve is identified as being that of a first-order-system. The relation between the time constant and the structural parameters is established, accounting for the non-linearity of the pilot hydraulic bridge. For the continual control of the stepper motor, a mathematical model considering the rotary motion, the rotating magnetic field and the angular control signal is established. In order to prevent the stepper motor from losing steps, the rate of the control signal is limited to a certain range. As a result, this may cause a non-linearity and, consequently, the deformation of the waveform when the input sinusoid wave is of large amplitude and high frequency. By utilizing the method of the description function, the effect of limiting the rate of the control signal is approximated as a first-order-system and the relation between the time constant and the amplitude and frequency is presented. The dynamic characteristics of the stepper motor are to a large extent dependent upon the way the power is supplied. For a constant current supply, the stepper motor can be classified as a second-order-system. The factors affecting the natural frequency and damping ratio are clarified. Finally, the frequency response of the 2D digital valve is experimentally measured and compared with theoretical results. Both theoretical and experimental results show that the 2D digital valve has a fairly high frequency response, especially when the valve operates near the central position. For a 25% full scale input signal, the 2D digital servo valve has at least 300 Hz under the gain of–3 dB.