The low bearing capacity and high crushability of coral sand pose major challenges to island and reef foundation engineering. Herein, a newly developed non-isocyanate polyurethane was used to reinforce coral sand and the unconfined compressive behavior of polyurethane-reinforced coral sand was investigated based on an orthogonal experimental design. The effects of particle gradation, the mass ratio of polyurethane to sand and moisture content on strength, deformation, and energy evolution were analyzed. The results show that the highest uniaxial strength of 5.48 MPa was obtained for naturally graded coral sand with a polyurethane mass ratio of 30% under dry conditions. The moisture content was identified as the dominant factor affecting the strength, and elastic modulus of the reinforced samples. Increasing moisture content significantly reduced the crack initiation stress, dilation strength, peak strength and elastic modulus, while increasing Poisson’s ratio. In contrast, a higher polyurethane mass ratio improved the strength, stiffness and energy dissipation capacity, whereas particle gradation primarily influenced the crack initiation stress level. Under unconfined compression, the reinforced samples mainly exhibited ductile shear failure involving edge breakage, particle sliding, delamination and rupture of the cured polyurethane film. Microscopic observation indicated that the cured polyurethane worked as a surface film, an interparticle bridge and a pore-filling phase within the coral sand matrix. The enhancement in mechanical behavior was mainly associated with polymer bridging, pore filling, local interfacial adhesion and mechanical interlocking.