To explore the water stability and durability of fly ash and slag geopolymer-solidified feldspar powder roadbed filler, a method combining experiments and numerical simulation was adopted. Based on the analysis of the influence of the mass ratio of fly ash and slag and the modulus of activator on the unconfined compressive strength of fly ash and slag geopolymer mortar and fly ash and slag geopolymer-solidified feldspar powder mixture, the fly ash and slag geopolymer-solidified feldspar powder mixture was prepared with the optimized mix ratio. The softening coefficient and dry-wet cycle tests were carried out, and the finite element model was used to analyze the influence of different dry-wet cycle times on the settlement of the fly ash and slag geopolymer-solidified feldspar powder roadbed under vehicle load. The results show that the dosage of geopolymer significantly affects the water stability and durability of the fly ash and slag geopolymer-solidified feldspar powder mixture. The softening coefficient increases from 0.77 to 0.81 with the increase in the dosage of geopolymer. The loss rate of strength of the fly ash and slag geopolymer-solidified feldspar powder mixture after 30 dry-wet cycles is 3.8%. The maximum settlement of the roadbed shows a nonlinear growth trend with the increase of dry-wet cycle times, and the settlement gradually decreases with the increase in depth. Due to the water absorption and expansion of clay minerals, which destroy the cementation between particles, the dosage of red soil has a significant impact on the cementation performance of the fly ash and slag geopolymer-solidified feldspar powder mixture.