Abstract: Fully focused phased array ultrasound theoretical research has been basically completed, and instrument system research and development has achieved results, but application research is relatively lagging. In this paper, the sound field characteristics and signal characteristics of the full focus phased array are studied. A new concept about the field and a new technical route of field measurement and field calibration are proposed to solve the problem that the full focus phased array system does not have A-scan detection. At the same time, it is pointed out that due to the unique sound field characteristics and signal characteristics of the full focus phased array, it has excellent performance in weld inspection.