Abstract: Based on the characteristics of the distance and moving velocity of the phase interface and the single-valued mapping between the moving-velocity and the output of controller, a new auto-control principle of the phase interface was presented. Firstly, the center between the initial position and target position of phase interface was chosen, then the position of phase interface was adjusted by means of the symmetry of the moving velocity of the phase interface, and finally the output value of the controller was determined at the moving velocity of zero. When the target position was nearly reached, the fuzzy control was chosen to adjust the position of phase interface. The principle was proved by both simulating and real experiment.