The polymer composites based on low-density polyethylene (LDPE), ethylene vinyl acetate copolymer (EVA) and fly ash (FA) without and with vinyltrimetoxysilan (VTMS) modification were prepared by melt mixing in a Haake Rheomixer. The FT-IR spectra indicated the bonds between LDPE/EVA and FA modified by vinyltrimetoxysilan were dipole - dipole interaction and hydrogen bonding. Thermal oxidation stability of the LDPE/EVA/VFA composites are higher than those of the LDPE/EVA/FA composites. Thermal oxidation stability of the LDPE/EVA/VFA composites and the LDPE/EVA/FA composites were higher than the LDPE/EVA blends. Putting fly ash into polymers, thermal oxidation stability of composites increased and significantly increased when using modified fly ash (MFA).