A servo hydraulic driving mechanism was designed using servo-tube control principle, this mechanism can move continuously and position accurately. The three-dimensional flow field of the drive cylinder is calculated and analyzed. The results indicate that the driving mechanism can effectively control the differential pressure of the system by changing the gap of the variable throttle orifice, the diameter of servo-tube is as small as possible. When the gap of the variable throttle orifice is less than a certain millimeter, the control effect is more obvious.