In order to study the effect behavior of internal pressure on the wall thickness thinning rate of thin-wall U-shaped bellows with small diameter, using the numerical simulation analysis method, the finite element model of internal pressure forming process for the thin-wall U-shaped 316L austenitic stainless steel bellows with small diameter was established based on ANSYS/LS-DYNA finite element analysis software. The distributions and evolution laws of equivalent stress, equivalent strain and wall thickness thinning rate during the internal pressure forming process of thin-wall U-shaped bellows with small diameter were obtained. The effect law of different pressures on the wall distribution of thickness thinning rate for thin-wall U-shaped bellows with small diameter under the same axial feeding displacement conditions was obtained. The forming internal pressure range of qualified forming thin-wall U-shaped bellows with small diameter was obtained. The research results provide the data reference to the design of internal pressure forming process for thin-wall U-shaped bellows with small diameter.