AbstractAir-hardening organic polymer (AHOP), a new soil stabilizer, can form a cross-link among soil particles with high strength and toughness. In this study, the effect of the polymer content and density on the mechanical behavior of stabilized silty sand was investigated. The results indicate that the unconfined compressive strength and tensile strength of stabilized silty sand keep a prominent linear relationship with AHOP content that increases with the increment in density; the increment in AHOP content also leads to a higher elastic modulus of stabilized silty sand. The cohesion was also significantly enhanced by the increase in AHOP content and density; the internal friction angle of loose specimens (ρ≤1.50 g/cm3) keeps increasing with AHOP content; however, for dense specimens (ρ≥1.55 g/cm3), 3% polymer content is a turning point, at which the internal friction angle changes nonmonotonically. AHOP film can connect loose silty sand particles and adhere to the surface of particles, leading to silty sand with good strength properties. Based on failure modes, the failure of polymer–particle interactions can be divided into four modes, including (1) tensile fracture at membrane, (2) tensile fracture at polymer–particle interface, (3) shear at membrane, and (4) shear at polymer–particle interface.