Abstract: The creep and progressive failure of loess induced by the time effect of the load can significantly affect the long-term service performance of its engineering structures. Especially in Northwest China,loess is widely distributed. Under the long-term action of the load,some typical engineering structures(such as the loess tunnel project excavated by Xi'an Metro) may be unstable due to loess creep and progressive failure,which can have an extremely negative impact on the smooth implementation of the project construction and its safe operation and maintenance after the completion of the project construction. It is of great theoretical and practical significance to analyze the rheological properties of loess and to reveal its rheological mechanism and then to establish its constitutive model. For this purpose,taking Q₃ loess as the research object,based on the fractional derivative theory,the typical creep deformation process is analyzed. The fractional element model that can simulate the non-linear change characteristics of the accelerated creep stage is proposed. After theoretical analysis,the fractional-order improved Nishihara model is established and the constitutive equation is derived. On this basis,in order to verify the effectiveness of the fractional-order improved Nishihara model,the original and reshaped samples of Q₃ loess triaxial hierarchical cyclic loading-unloading rheological test is carried out. The corresponding theoretical calculation results show that the fractional-order improved Nishihara model not only can simulate the three rheological stages of Q₃ loess deceleration creep,constant velocity creep and accelerated creep,but also can make up for the inability of the integer-order improved Nishihara model to describe the accelerated creep stage. Compared to the integer-order improved Nishihara model,its prediction effect is better in the deceleration creep and unloading stages. The research results provide an important theoretical basis for the safe operation and maintenance design and construction plan of buildings considering the rheology of loess.