Based on the real exposure field environmental conditions in the field, this paper investigates the strength evolution of concrete in long-term service under the coupling of corrosion-freeze-thaw-large temperature difference in saline soil area, and reveals the intrinsic mechanism of the change of concrete mechanical properties through NMR, XRD, FTIR, SEM-EDS tests. The findings of this study demonstrated that adding fly ash (FA) and slag powder (SP) to concrete could effectively improve its physico-mechanical properties after six years of corrosion in saline soil. The concrete incorporated with 10% SP exhibited the lowest surface damage and the greatest compressive strength. The internal porosity of concrete with 30 % FA, 10 % SP and 20% SP is less than that of normal concrete. These changes optimised the pore structure and supported the idea of an intrinsic strength-evolving mechanism. The crystalline expansion of sodium sulphate is an important factor leading to the decline in concrete performance in saline soil areas. Mixing 30% FA, 10% SP, and 20% SP promoted the hydration of concrete to generate CSH and calcite generated by the carbonation of CH fills the pores in the concrete, increasing the concrete compactness and strength and further preventing the infiltration of SO42-and Cl-. This also reduced the generation of corrosion products such as ettringite, gypsum, thaumasite, and others. By analysing the macro-performance changes and micro-mechanisms, it was found that 10% SP internal mixing demonstrated the best effect on the performance enhancement of concrete piles in a real environment in saline soil areas for up to six years.