In an alpine saline soil area, the combined effect of freeze–thaw and erosion ions causes the imbalance of concrete durability. In studying the macro–meso deterioration law and failure mechanism of polyacrylonitrile fiber-reinforced concrete (PANFC) under this action, PANFC specimens formed by open-air curing were prepared, and a composite salt solution containing 5% MgSO4, 5% Na2SO4, and 3.5% NaCl was used as the freeze–thaw medium for salt-freezing test. Results demonstrate that the degradation of PANFC under salt-frost conditions can be divided into the micro-damage stage and damage acceleration stage, with 50 cycles as the limit. When the content of PAN fiber is 1.5–1.8 kg/m3, the salt frost resistance of concrete is advantageous. The macro-scale results show that the relative dynamic elastic modulus and ultrasonic wave velocity of PANFC decrease with the increase of salt-freezing cycles, whereas the weight of PANFC increases before 25 salt-freezing cycles. The meso-scale results show that after 50 cycles, the evolution of micro-pores to macro-pores and micro-cracks in PANFC is accelerated. The pore size ratio of micro-pores (0–0.1 μm) decreases with the increase of cycles, and the pore size ratio of micro-cracks (greater than10 μm) increases with the increase of cycles. Failure mechanism analysis shows that the failure of PANFC results from the combined effect of surface damage and internal damage. Finally, the salt-frost damage model of PANFC was established on the basis of the damage mechanics and classical Aas-Jakobsen fatigue theory, and the damage determination value Dn was between 0.2527 and 0.2781. Using Dn, the failure criteria of concrete damaged under salt-frost conditions can be re-evaluated.