The popular sensors used in land welding automation are faced with lot of difficulties while they are used in water. It is meaningful to find a new kind of sensor which is suitable for wet welding. Ultrasonic Phased Array(PA) can work in water conveniently, which controls each element to send out required sound beam separately and focus on specific points. Compared with single ultrasonic sensor, PA works fast and effectively. Interference principle of acoustic beam is analyzed at first, and then the relationship between focusing precision and the shape of PA, the number of sensor element, gap distance between adjacent units and time resolution is revealed with simulation. Then, proper physical parameters of PA are determined. Finally, high time -resolution circuit based on CPLD is given out, which works together with sound emitting and receiving circuits to realize fast scan on welding workpiece, so that the seam line could be deduced with underwater distance detection. Researches manifest that PA is effective for wet welding.