According to the freeze-thaw performance of high-speed railway subgrade filler in a seasonal freezing area, graded crushed stone mixed with cement and fly ash is used as improvement measures. In order to study the anti-freeze-thaw durability of cement and cement fly-ash improved filler, the water stability, freeze-thaw deformation characteristics, and freeze-thaw durability of the filler are tested. The influence of the modifier’s type, dosage, and curing age on the performance of subgrade filler is analyzed. The results show that the cement-modified filler has higher unconfined compressive strength than the cement fly-ash-modified filler. The unidirectional frost heaving of filler decreases with increasing cement content, and the frost heaving rate of filler increases with the addition of fly ash. After 28 days of curing, the water stability of cement fly ash-modified filler is better than that of cement-modified filler, and the freeze-thaw resistance is lower than that of cement-modified filler.