Abstract: Researches were conducted on the influence of the variable load on the fatigue life of the economizer inlet pipeline structure in a 600 MW thermal power unit, the relationship between strain and fatigue life was obtained through following three methods, such as material tensile test, fatigue test, and numerical simulation method. Results show that, the fatigue life of material tensile test is quite different from that obtained by fatigue test, while the fatigue life of numerical simulation method is in accordance with that of fatigue test. The fatigue performance of the structure can be reasonably evaluated by numerical simulation method, while the life loss of the structure significantly increases with the increase of the change range of the unit load.