<p>To investigate the mechanical properties of red sandstone under cyclic loading, triaxial variable upper and lower limit constant amplitude cyclic loading and unloading tests were conducted on a TFD-2000 microcomputer servo rock triaxial testing machine under different confining pressures. The deformation characteristics and energy evolution mechanism of red sandstone were analyzed and discussed. A rock strength failure criterion was established based on energy mutation. The results show that the deformation modulus and plastic strain fluctuate significantly near the cycles preceding failure, indicating the impending instability of the red sandstone. The energy density of rock specimens increases with increasing confining pressure. In the elastic stage, the energy density of loaded rock specimens increases with the number of cycles, with a relatively large proportion of elastic energy density. The proportion of dissipated energy gradually increases in the cycles leading to failure. The energy consumption ratio can be used to characterize the internal damage accumulation of loaded rock specimens. Except for the first cycle of each stress ratio, the energy consumption ratio changes smoothly in other cycles at low stress levels. An inflection point appears before specimen failure, with a sudden increase in the growth rate, leading to a mutation. The energy consumption ratio decreases with increasing confining pressure.</p>