Abstract: In order to enlarge the usage extent of roadbed fillings for high-speed railways, dynamic tri-axial tests are made on cement- and lime-improved soil with different blend ratios subjected to freeze-thaw cycles. The variation and influence factors of critical dynamic stress of improved soil are studied. The test results show that the improvement effect is superior to that of lime soil under repeated freeze-thaw. Compared with those of clay, their mechanical properties are all improved visibly. In terms of resisting freeze-thaw, there is an optimal blend ratio. Meanwhile, the critical dynamic stress attenuation coefficient subjected to freeze-thaw cycles is introduced and applied in the design of improved soil proportion, aiming to choose rational method and proportion.