Abstract: In order to investigate the synergistic effect and fire resistance mechanism of Trimer combined with Zinc borate (ZB), the intumescent flame retarded epoxy coating was prepared using tri(2,6,7-trioxa-1-phospha-bicyclo 2.2.2 octane-1-oxo-4-methanol) phosphate (Trimer) as flame retardant and ZB as synergistic agent. The tests of fire resistance and cone calorimeter demonstrated that appropriate amount of ZB could exert a good synergistic effect. The fire resistant time increased from 17 min to 30 min, the peak-heat release rate decreased from 379.8 kW/m² to 258.5 kW/m², and the peak-smoke release rate reduced from 0.11 m²/s to 0.05 m²/s. The results of thermogravimetric analysis (TGA), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) show that the good fire resistance is related to the increase of the residual char, the formation of thermal stable BPO₄ within the residual char and the improvement on uniformity and compactness of the intumescent foamed char.