Abstract: In nuclear power plants, the flow accelerated corrosion （FAC） occurs mainly in the secondary circuit, causing a significant impact on production practice. Straight pipes with an orifice and jet impingement were simulated by CFD at 250 ℃ and 273 ℃, achieving flow patterns and shear force distributions on the surfaces of plates and in the internal of pipes, and identifying weak points caused by pipe thinning with an orifice and influence of jet impingement angle. The results show that the orifice diameter ratio in the straight pipe changing has little effect on the maximum point of shear stress. With smaller crashing angle, the greater the shear force is, and the stronger the mechanical action is.