To economically reduce seismic liquefaction damage to saturated loess foundations, variations in the characteristics of dynamic stress, dynamic strain, and dynamic pore water pressure are analyzed using dynamic triaxial tests on improved saturated loess with different dosages of fly ash. The law of influence of this dosage on the liquefaction stress ratio, dynamic residual deformation, and dynamic pore water pressure of the improved loess is then studied. Combined with microscopic test results, the physical-chemical anti-liquefaction mechanism of saturated loess improved by fly ash is also discussed. Results show that the dosage of fly ash has a significant influence on the liquefaction stress ratio, dynamic strain, and dynamic pore water pressure of the improved loess. With an increase in the dosage, there is a respective increase in the liquefaction stress ratio of the improved loess:after the dosage reaches 15%, there is an evident increase in the liquefaction stress ratio with a continued increase in the dosage. In addition, the dynamic strain and dynamic pore water pressure of the improved loess increase with a decrease in the fly ash dosage:after the dosage reaches 25%, the saturated improved loess cannot be liquefied. Scanning electron microscope (SEM) results indicate that the physical-chemical anti-liquefaction mechanism of saturated loess improved by fly ash is mainly related to the hydration process of the fly ash with respect to the filling effect of the colloid product and particles and adsorption of free water by fly ash.