Abstract: The porous structures of loess geomaterials exhibit heterogeneous, affecting its mechanical properties and therefore the stability of slopes when involving its geotechnical applications. However, the current research on the mechanical properties of heterogeneous loess bodies and on their influence on the formation and evolution of slope disasters remains insufficient. Therefore, in this study, an alternative elasto-viscoplastic constitutive model is proposed to characterize the stress-strain relationships of loess. In the constitutive modelling, the random spatial distribution function is implemented to represent the distributions of the porosity field, meanwhile, stiffness (elastic modulus) and strengths (cohesion) are coupled with porosity, to achieve the heterogeneous properties of mechanical behaviour. Furthermore, an innovative version of the strength reduction method has been proposed, which can be used to evaluate the factor of safety of heterogeneous loess slopes. Finally, comprehensive parametric analyses are performed to investigate the effect of main factors, including the heterogeneous parameters and slope angles, on the factor of safety and on the development of sliding regions (area and depth) of loess slopes. The research has demonstrated that the heterogeneity of loess significantly influences the stability of loess slopes. Additionally, the safety factor exhibits a quasi-linear relationship with the residual tangent value of the slope angle. The achievements in this study can contribute to a better understanding of the generation and growth of damage responses of heterogeneous loess slopes, benefiting the economic development as well as migration of disasters in the loess regions of China.