Syntactic foam with fly ash cenospheres as the main component has high specific strength and specific energy absorption, showing its great application potential in lightweight impact resistant structures and buffer protection.However, the influence mechanism of the factors such as the fly ash cenosphere size and the reinforcement on the mechanical properties and behaviors is still unclear.In addition, the mechanical model of this kind of syntactic foam has not yet been constructed in current studies, which is not conducive to the material selection and numerical simulation related work in structural design.With regards to this, a series of quasi-static compression experimental studies were conducted about the influence of the fly ash cenospheres size and the honeycomb aluminum reinforcement on the mechanical properties and deformation behaviors of cenospheres polyurethane syntactic foams (CPSFs).Meanwhile, the mechanical model of CPSFs was constructed according to the Avalle theory.The results are as follows: ① When the relative density is less than 0.29, the size of the cenosphere has little effect on the mechanical properties of the composite foam.When the relative density is greater than 0.29, the effect of the cenosphere size on the mechanical properties of the composite foam increases as the density increases.② For the composite foam containing reinforcing phase, the mechanical properties of the composite foam containing small-sized cenospheres are significantly improved.The additional reinforcement effect of the aluminum honeycomb is more obvious for the composite foam containing small-sized floating beads.This reinforcement mechanism is mainly to change the initial failure mode of the material from shear to axial crushing.③ The results by the constitutive model constructed according to the Avalle theory, the fitting of the stress platform stage and the energy dissipation characteristics are consistent with the experimental results, and the basic mechanical properties of the material can be predicted more accurately.The study provides a theoretical reference and a basic prediction model for the comprehensive utilization of fly ash and promotes the application of its composite foam in the design of lightweight impact-resistant structures.