Abstract: Fissured loess is one of the important factors causing geological disasters in the Loess Plateau region. Exploring the evolution law of mechanical properties and deformation development of fissured loess under true triaxial conditions is of great significance for disaster prevention and control and engineering construction safety in the Loess Plateau region. To study the evolution law of mechanical properties of fissured loess under true triaxial conditions, this paper conducts true triaxial tests on fissured loess with different fissure angles and analyzes the variation law of mechanical properties of fissured loess under different confining pressures and different b values. The results show that: (1) The peak axial stress of loess specimens with different fissure angles shows three different development patterns: gradually increasing, first decreasing and then increasing, and gradually decreasing. (2) Under confining pressures of 100 kPa with b=0.50, 0.75 and 1.00, and 200 kPa with b=0 and 0.25, the axial stress-strain curves of 90° fissured specimens show inflection points. (3) The principal strains of loess specimens with different fissure angles can be divided into three development stages: shear contraction development, stable development and shear expansion development with the development of the major principal strain. At the same time, the shear expansion stage becomes shorter with the increase of confining pressure. (4) When the fissure angles are the same and b values are small, with the increase of generalized shear stress, the intermediate principal strain shows a shear expansion phenomenon, and the larger the fissure angle, the more obvious the shear expansion. The research results can provide a reference for the prevention and control of engineering disasters and stability evaluation of fissured loess.