Abstract: The low-cycle fatigue life of alloy steel is the critical factor affecting its safe application. The low-cycle fatigue properties of K4 alloy steel at room and high temperatures were investigated. The test was carried out using low-cycle fatigue test method controlled by strain at room temperature, 650 ℃, 700 ℃ and 725 ℃ for alloy steel according to GB/T 15248 standard. The strain-life datas were fitted by the Coffin-Manson formula, and the low-cycle fatigue design parameters of the material were obtained. Meanwhile, stable hysteresis-loop curves at different strain levels were provided. The low-cycle fatigue fracture morphology was analysed by environmental scanning electron microscope(SEM). The results show that the low-cycle fatigue life decreases overall with temperature increasing. The Coffin-Manson formula achieves the best fitting effect for the strain-life data of the alloy steel at room temperature. The alloy steel exhibits mainly plastic deformation during low-cycle fatigue at room temperature, with elastic deformation predominate after the transition fatigue life at 650 ℃, 700 ℃, and 725 ℃. The fatigue life generally increases with temperature increasing under the same elastic strain level. However, the fatigue life decreases with increasing temperature under the same plastic strain level. A comparison of stable hysteresis loops indicates that the plastic deformation of the alloy steel enhances with increasing total strain amplitude. Fatigue fracture show that the low-cycle fatigue of this alloy steel is primarily characterized by fatigue fracture, with static load transient fracture features appearing at a larger total strain amplitude at 650 ℃. Oxidation becomes severe and plastic deformation dominant as the temperature increase. The width of the fatigue bands increases with the total strain amplitude increasing. The fracture surface is rougher at higher strain levels, exhibiting a tearing-like dimple morphology.