Abstract: In seasonally frozen regions, the dynamic properties and microstructure of fill-solidified lightweight soil undergo significant changes under freeze–thaw cycles. To evaluate its feasibility as subgrade filler, a series of dynamic triaxial tests simulating traffic loads and microstructural analyses were conducted under varying freeze–thaw cycles, freezing temperatures, and loading conditions. Correlation analysis was further carried out between microstructural parameters and dynamic properties. The results show that freeze–thaw cycles have a pronounced effect on the dynamic behavior of solidified lightweight soil. With increasing cycles, the critical dynamic strength gradually decreases, and the rate of reduction slows, indicating that the freeze–thaw damage effect tends to stabilize after repeated cycles. Moreover, the critical dynamic strength decreases with lowering freezing temperature. When the temperature falls below −10 °C, the reduction becomes markedly more severe, indicating the presence of a temperature sensitivity threshold beyond which structural damage is markedly intensified. Based on the evolution of critical dynamic strength, a predictive model was established and verified. Microstructural analyses reveal that freeze–thaw cycles increase the number of pores and cracks, with small- and medium-sized pores evolving into larger ones, while pore shapes tend to become more elongated and angular. Among the microstructural parameters, porosity presents the strongest influence on dynamic properties. In summary, fill-solidified lightweight soil demonstrates degradation in dynamic performance and microstructural integrity under freeze–thaw and low-temperature conditions. These findings provide a theoretical basis for its rational application in subgrade engineering.