Abstract: In the drill-and-blast construction of tunnels under high geotemperature conditions, the mechanism governing damage evolution in early-age Steel Fiber Reinforced Shotcrete (SFRS) subjected to combined thermal and mechanical loads has not been fully understood. A damage constitutive model was developed utilizing the Split Hopkinson Pressure Bar (SHPB) technique and the fractal dimension analysis of CT images. This model facilitated the study of the dynamic response and damage features of SFRS specimens (aged 1 to 3 days) subjected to impact loading, following a curing regime involving high and fluctuating temperatures. The results indicate that the high-temperature varying-temperature curing environment markedly degrades the dynamic mechanical properties of SFRS. The reductions in dynamic strength were measured at 16.1%, 38.1%, and 56.5% for specimens aged 1, 2, and 3 days, respectively, demonstrating a cumulative temperature-induced damage effect with increasing age. The 3-day-old SFRS demonstrated the highest energy absorption efficiency, with its proportion of dissipated energy increasing by 98.9% and 16.7% relative to the 1-day and 2-day-old specimens, respectively. The developed damage constitutive model yielded a goodness-of-fit exceeding 0.9, proving capable of representing the progression of the stress-strain curve for SFRS under impact loading across its elastic, yielding, and failure phases. Furthermore, the research reveals the dynamic damage evolution law of early-age SFRS under thermo-mechanical coupling, which can provide a theoretical basis for the design of support structures in high ground-temperature tunnels.