###### Abstract
[24] studied the volumetric variation and hydraulic properties of unsaturated soil at different water contents and found a logarithmic relationship between the hardening coefficient and saturation of unsaturated soil. In the meantime, the soil is a typical structural material, and strain hardening directly affects its strength and deformation. Therefore, constitutive models of unsaturated soil have been developed from the perspective of damage. [PERSON] et al. [26] concluded that the hardening and damage of soil were attributed to the closure and expansion of pores and established a damage model for frozen soil. [PERSON] et al. [27] developed a soil damage constitutive model considering suction effects, plastic flow, and damage based on the continuous damage theory. [PERSON] et al. [28] adopted the novel electrical impulse method and water-air transport laws to develop a percolation model for collapsibility elastoplasticity damage that considered the loess structure based on the Barcelona model. Moreover, it has been found that fractional calculus is a powerful tool for modeling the viscoelastic/viscoplastic behaviors and is particularly suited for building a time-dependent constitutive model [29; 30; 31; 32; 33]. Based on fractional calculus, [PERSON] et al. [31] presented the fractional Bingham model that can describe the time dependent behavior in muddy clay with yield strength. By replacing a Newtonian dashpot in the classical Nishihara model with the fractional derivative Abel dashpot, [PERSON] et al. [32] proposed a new creep constitutive model for salt rock on the basis of a time-based fractional derivative. Using experimental data from several tests, such as creep and creep recovery, that were performed at different temperatures and at different stress levels, [PERSON] et al. [33] presented a fractional viscoelastic and viscoplastic model of asphalt mixtures. With various angles, reasonable methods, clear patterns, and reliable conclusions, research on the mechanical properties of unsaturated soil provided theoretical and technical support to the relevant engineering projects and strongly promoted the development of unsaturated soil mechanics. However, deficiencies in the study of mechanical properties of unsaturated soil are non-negligible [34]. For example, relatively few studies have focused on the laws of mechanical properties, viscoelasticity, and damage evolution of unsaturated soil under the coupling effect of hydraulic properties and stress, and no consensus has been reached. In the meantime, these properties are the key to solving the strength and deformation problems in regions with unsaturated soil. Therefore, it is necessary to further study the mechanical properties of unsaturated soil. Based on the basic theories of soil mechanics, fractional-order calculus and statistical damage principle, a fractional-order damage constitutive model of coupled moistening fields and stress fields is established by determining the effective stress, shear strength expression, viscoelastic expression, and damage variable evolution formula of unsaturated soil. The hydraulic effect, strength law, viscoelastic law, and damage evolution law of unsaturated soil in moistening fields and stress fields are further analyzed through the discussion of model parameters. This study provides a new model to describe the mechanical mechanism of strain hardening of unsaturated soil under the influence of water and stress.