Abstract: Piping transportation of supercritical carbon dioxide (CO₂) is a key link connecting the global Carbon-Capture-and-Storage (CCS) plan aiming to reduce excessive emissions of greenhouse gases like Co₂. Therefore, it is of utmost importance that a series of problems involving the growth paths of fatigue cracks in CO₂ pipelines with crack-like surface defects are taken into account when the leak-before-break (LBB) fracture criteria is used to evaluate the safety of pressure piping. Based on the linear elastic fracture mechanics (LEFM) theory and the ABAQUS software platform, this paper builds a three-dimensional finite element model of a pipe segment containing an axial semi-elliptical inner surface crack, which can be used to obtain the fracture parameters of the surface crack under axial forces, bending moments, and/or internal pressures by adopting the virtual crack closure technique (VCCT). On the platform of this presented finite element model in fracture mechanics, focusing on two types of thin-walled CO₂ pipelines (thickness-radius ratios: t/R=1/10 and 1/27.3) under their specific operation pressures, this paper mainly discusses fatigue propagation problems of an axial semi-elliptical in-wall surface crack with different initial aspect ratios(α₀1) caused by pressure fluctuation. Specifically, the fatigue propagation paths of the six inner surface cracks are analyzed when their initial aspect ratios are chosen as 1, 2/3, 1/2, 1/4, 1/6 and 1/8, respectively. The fatigue propagation stages for these cracks are drawn and their fatigue crack growth lives are compared. Meanwhile, the effect of thickness-radius ratios on the crack growth trends is also discussed. The numerical results show that the fatigue growth paths of these cracks have a close relationship with their initial geometrical shapes. There also exists an asymptotic route for fatigue propagation of these cracks, which is closely connected with the thickness-radius ratio of pipeline.