Abstract: During the flattening test of large-diameter Ti-4Al-3Mo-1V titanium alloy seamless pipes, cracking occurred in the inner wall of the flattened rings. The factors affecting the flattening performance of large-diameter titanium alloy pipes were studied by finite element simulation and microhardness testing. The results show that cracks are inclined to form at the positions of 6 oʼclock and 12 oʼclock of the inner surface of the flattened rings due to higher tensile stress. The main reason for the flattening cracking of the pipes is that the oxide film formed on the surface of the pipes during heat treatment, leading to the increase of surface hardness, which in turn resulted in local stress concentration. Removing the hardened layer on the pipes’ surface through acid washing can avoid cracking during flattening.