In order to enhance the mechanical properties and durability of tunnel shotcrete, a high-performance shotcrete admixture (referred to as high-performance admixture) for tunnel shotcrete was developed in this paper. By means of hydration heat, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), the mechanism of high-performance admixtures to improve the mechanical properties, rebound rate and durability of shotcrete was studied. A predictive model for relative dynamic elastic modulus was established based on the number of freeze-thaw cycle and high-performance admixture content in shotcrete. The results show that the high-performance admixture promotes cement hydration process, increases hydration product formation, and refines pore structure in hardened cement, resulting in denser microstructure that enhances both strength and durability. When the content of high-performance admixture is 10% of the mass of cementitious material, the compressive strength of shotcrete increases by 51.7% and the rebound rate decreases by 54.5% compared with the reference group.