<p>A large-caliber gun barrel is a typical thick-walled cylinder which is designed to fire hundreds or thousands of projectiles all along its total life. Generally, a lot of micro-cracks generate on the gun bore and propagate outward along the wall thickness direction under repeated internal pressure loading. However, the micro-cracks can also occur on the outer surface due to machining, corrosion, unexpected damage, etc. When a gun fires, the tensile hoop stress induced by the internal pressure will make the micro-cracks propagate inward from the outer surface. Furthermore, the tensile residual stress on the outer surface induced by autofrettage of gun barrel will promote the growth of micro-cracks. In this study, a novel self-designed test rig was used to investigate the crack initiation and propagation of thick-walled cylinder specimens under cyclic internal pressure experimentally. The circumferential (hoop) strain on the outer surface was monitored continuously during the fatigue testing process and used as the health index of the tested specimens. An exponential function model was established to describe the relationship between the circumferential strain and fatigue cycles. The suppression of the detrimental effect of the tensile stress on crack initiation and propagation was also discussed. The results highlight the importance of proper design, manufacturing and maintenance of gun barrels in order to prevent shortening of their total fatigue life.</p>