Expandable Graphite (EG) and sepiolite were added to the traditional APP/PER/MEL systems to get a new kind of water-based intumescent flame retardant coatings for steel structure. The effects of EG/ sepiolite on the fireproofing time and char formation of the coatings were investigated by using heat insulation test, thermo gravimetric analysis (TGA), derivative thermogravimetry (DTG) and X-ray diffraction (XRD). The results of TGA and DTG showed that the use of EG and sepiolite together could exert the flame retardant synergy effects: EG could postpone the formation of char layer at quite low temperature zone while sepiolite increased the stability of the residual char at higher temperature. XRD analysis indicated that the performance of anti-oxidation of the coatings was improved by adding EG and sepiolite. In addition, the fireproofing time of the steel structure was up to 72min when the film thickness of the coatings, which contained 3wt% sepiolite and 2%EG, was 1mm.