Abstract: Bolts are widely used for the connection of components in such as thermal power and wind power. They are subjected to complex stresses and temperatures during service and are likely to cause bolt breakage or failure. Their safety has received more and more attention. How to detect the fatigue crack of the bolt without its removing is always a problem. In this paper, combined with theoretical analysis, the key technologies of guided wave phased array and ultrasonic phased array detection of bolts are studied respectively. The in-service bolt detection process and scheme are developed to realize the full coverage of in-service bolt detection and completely solve the in-service bolts detection problems.