Abstract: Calibration of the detection system is an essential step in implementing ultrasonic testing. In the past, the calibration of various ultrasonic detection technologies was based on A-scan by following many prescribed steps, and is time-consuming and of poor accuracy. The "field calibration" technical route proposed by studying the ultrasonic field characteristics and signal characteristics of a fully focused phased array has a series of advantages such as easy-to-understand principles, simple processes, fast operation, high accuracy, and high degree of automation. This article describes the principle and basic process of field calibration, and gives some test results. It is pointed out that the application of field calibration can greatly simplify the operation of ultrasonic inspection of welds with high accuracy, which can comprehensively enhance the advantages of full focus phased array technology.