Abstract: The welding forms of the stiffening ring and pipe wall of penstock include the butt weld and fillet weld. The fillet weld is more widely used in practical engineering because of its low welding difficulty.However, scholars often simplify or directly adopt the form of the butt weld in the simulation calculation of external pressure resistance stability of penstock, ignoring the impact of fillet weld connection, which is inconsistent with the engineering practice and causes some errors. In this paper, the three-dimensional finite element software ANSYS is used to simulate the force transfer mechanism of penstock and stiffening ring when fillet weld and butt weld are used, the influences of the two weld forms on the external pressure stability of penstock are compared, and the influences of different leg heights of the fillet weld on the structural stability of penstock are explored in order to provide some reference and basis for the actual construction of the project.