In order to investigate the dynamic characteristics of fly ash mixed with different amounts of fly ash and the modification mechanism of fly ash improved fly sand soil, Suihua-Daqing Expressway roadbed fill was taken as the research object. The lateral limit compression test and dynamic triaxial test were conducted to analyze the influence of fly ash mixture on the static mechanical characteristics of fly sand soil, as well as the influence law of surrounding pressure, dynamic load loading times and fly ash mixture on the dynamic strength of improved fly sand soil. The results show that the unconfined compressive strength and dynamic strength of the soil, with the same circumferential pressure, both increase firstly and then decrease with the increase of fly ash admixture. For the same number of loadings, the dynamic strength of the soil increases with increasing circumferential pressure. The loading number of 100 times of dynamic strength is selected for fitting analysis, and the empirical equation corresponding to the relationship between fly ash admixture and dynamic shear strength is derived. In order to improve the strength of the roadbed and speed up the process of comprehensive utilization of fly ash, the test results were combined with data analysis. It is recommended to mix the quality ratio of 15% fly ash for roadbed construction in the pulverized sand soil roadbed. The improved chalk soil with 15% fly ash admixture has the advantages of compact particles, smaller pores, relatively dense structure and stronger skeleton compared with the plain soil, which can provide reference for the construction of chalk soil improvement and other projects in northeast China.