Abstract: In this paper, the thermal fatigue of thick wall metal parts caused by flexible operation mode of thermal motor unit is studied. Taking the field defect inspection and welding repair project of high-pressure main steam valve of a power plant as an example, the stress field in thick wall metal parts of thermal power unit under peak loading condition is analyzed by finite element method. It is found that the valley point of alternating stress amplitude can be formed at a certain depth of the wall thickness of the main steam valve during peak loading operation of thermal power units. The occurrence mechanism of the valley point and the inhibition effect on crack initiation and propagation are studied, which provides a theoretical support for the failure analysis and defect treatment of thick wall metal parts. Valley point is the depth position where the alternating stress amplitude decreases from the maximum value of the component inner surface to the minimum value with the increase of the depth from the inner surface. Therefore, the microcracks formed within the depth of the valley point will preferentially expand to the inner surface under the action of the valley point, and it is difficult to form new fatigue cracks after exceeding the valley point. The above research results are verified by analyzing the crack position and depth distribution found in the actual detection of the main steam valve. In addition, by replacing the valve model with the model of a section of steam pipeline and analyzing the stress field under peak loading conditions, it is found that the formation position of valley point is related to the geometric size and heat dissipation conditions at the outer wall, and has nothing to do with the density, thermal expansion coefficient, thermal conductivity, specific heat capacity, Young’s modulus of elasticity and other properties of component metal materials. The above research results have guiding significance for further understanding and further reducing the thermal fatigue damage of metal parts with thick wall characteristics caused by flexible operation of thermal power units.