To meet the construction requirements in the desert zone, the consolidated undrained (CU) triaxial compression tests and scanning electron microscope (SEM) analysis were carried out, and the effects of mass fraction of fines and geopolymer (GP) content on the shear strength properties and microstructure of aeolian sand were explored based on the critical state theory. Results show that with an increase in the mass fraction of fines, the stress-strain curve of aeolian sand shifts from softening type to hardening type, and the peak deviatoric stress decreases. The peak and residual deviatoric stress of stabilized aeolian sand both increase with increasing GP content, and the stabilized aeolian sand shows obvious brittle failure and dilatancy. The inclusion of fines into aeolian sand benefits the increase of cohesion and reduces the internal friction angle. The cementing effect of GP significantly enhances the cohesion of stabilized aeolian sand and has little effect on the internal friction angle. The critical state lines of stabilized aeolian sand and untreated aeolian sand are almost parallel. For stabilized aeolian sand with a 20% mass fraction of fines, its critical state line is located beneath the one without stabilization and gradually moves downward with increasing GP content. For stabilized aeolian sand with a 30% mass fraction of fines, its critical state line is located above the one without stabilization and gradually moves upward with increasing GP content. SEM analysis results indicate that the inclusion of fines fills the pores and changes the contact pattern between aeolian sand particles, which effectively increases the compactness and provides cementing areas for GP. The combined effect of fines and GP significantly increases the shear strength of stabilized aeolian sand.