To reveal the instability failure mechanism and energy evolution law of the rock mass with multiple cracks, a numerical model of red sandstone was established by using PFC2D. The mesoscopic parameters of the numerical model were calibrated based on the results of uniaxial compression tests and Brazilian splitting tests of intact red sandstone specimens. On this basis, the particle flow simulation tests of red sandstone with multiple cracks were carried out. The results show that with the increase of λ, the peak strength of the multiple-cracked red sandstone gradually decreases when the inclination angle remains unchanged; with the gradual increase of the inclination angle α, the peak strength of the multiple-cracked red sandstone gradually increases when the short-long axis ratio λ remains unchanged; the failure mode shows a diagonal tensile-shear failure, with tensile as the main and shear as the auxiliary, and the failure and instability mode of the specimens are all along the extension direction of the prefabricated cracks; the failure mode of the multiple-cracked red sandstone is jointly affected by the short-long axis ratio and the inclination angle; before the peak strength, the rock mainly shows the energy accumulation characteristics; at the peak strength, the total energy of the red sandstone is mainly elastic energy and supplemented by dissipated energy; at the instability failure, the total energy of the red sandstone is mainly dissipated energy and supplemented by elastic energy; the energy storage capacity and failure difficulty of the red sandstone change with the variation of the multiple-crack inclination angle α and the working conditions, which can be used as a reference for rock breaking operations; and a high-precision damage constitutive model of red sandstone based on different height-diameter ratios λ and different joint inclination angles α was established.