Abstract: The initiation and propagation of fatigue cracks inside the gun barrel wall are objective phenomena, leading to potential firing safety hazards throughout the entire service life of the gun barrel. Occasional barrel burst accidents often endanger the safety of personnel and equipment. In engineering practice, endoscopes or other optical systems are usually used for qualitative evaluation of surface damage inside the gun bore, or diameter gauges are employed to measure the diameter variation at different axial positions of the barrel. Neither of these two methods can accurately locate and monitor the fatigue cracks inside the barrel wall and their propagation. To address this problem, this paper develops a composite non-destructive testing method combining ultrasonic phased array and strain monitoring. Specifically, ultrasonic phased array is first used to locate severely damaged areas on the inner wall of the barrel, and then strain gauges are attached to the corresponding outer wall surface of the barrel to monitor circumferential and axial strains for tracking crack propagation. Meanwhile, a hydraulic fatigue test device for simulating the thick-walled cylinder structure of gun barrels is independently designed, and experimental verification is carried out using the MTS Landmark 370.50 fatigue testing machine. The results show that the ultrasonic phased array testing can rapidly locate macroscopic damage zones on the inner wall, while strain measurement techniques can accurately capture the propagation of internal micro fatigue cracks. The circumferential and axial strain test data acquired from the outer surface of the gun barrel, together with numerical simulation results, well reflect the inherent mapping relationship between strain variation and crack growth. The combination of ultrasonic phased array inspection and strain monitoring can provide scientific, reliable and practical technical support for barrel health assessment and the prevention of catastrophic firing safety accidents such as barrel burst.