Understanding the mechanical behaviour and microevolutionary trend of deep loess is important for future engineering construction of high slopes and deep foundations on the Loess Plateau. True triaxial compression tests and scanning electron microscopy (SEM) imaging of deep loess samples were conducted. The microscopic properties of loess pores in the obtained SEM images were identified using Particle (Pore) and Crack Analysis System software. The relationships between the microparameters of the pores and mechanical properties of loess were explored using Pearson's correlation analysis. The results of this study demonstrated that the critical intermediate principal stress parameter bs is related to the soil properties and loading path and is independent of the confining pressure. The strength of deep loess linearly increases with confining pressure, and the SMP strength criterion provides the most accurate prediction of the shear strength parameters of Xi'an loess. Among the deep loess samples, the proportion of medium pores is the highest. The directional behaviour of the pore area is weak, and an increase in the confining pressure does not significantly affect the pore area. However, this process leads to a more disordered arrangement of pores. Furthermore, the mesopore content and confining pressure, as well as the orientation probability entropy of the number of pores, play a positive role in the strength of deep loess, while the total pore and macropore contents negatively impact the strength of deep loess. The fractal dimension, micropore content and orientation probability entropy of the pore area are not significantly correlated with the strength of deep loess.