It is well accepted that lower hybrid current drive (LHCD) is the most efficient method for non-inductive current drive in fusion devices and the effect of the current drive is dependent on not only microwave power but also its grill phase shift. This paper presents a new kind of feedback control system for antenna phase difference in LHCD experiments. In this high-speed control system, a lot of new technologies and methods are incorporated. The results of the experiments show a very good agreement with the system design.