A boric acid modified expandable graphite (EG(B)) was prepared through oxidizing-intercalating reaction of natural graphite, using H2SO4 and boric acid (H3BO3) as intercalators simultaneously. The dilatability of the intercalated products were characterized with expansion volume and initial expansion temperature. Scanning electron microscope, X-ray diffraction spectroscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy were employed to detect its layer structure, main element relative contents and function group. At the same time, its influence on combustion characteristics and thermal stability for linear low density polyethylene (LLDPE) were investigated in limiting oxygen index (LOI) tests, vertical burning tests and thermal gravimetric/differential thermal gravimetric analyses. Comparing with the normal expandable graphite (EG, intercalated by single H2SO4), EG(B) exhibited high dilatability, thermal stability and flame retardancy for LLDPE. It had been testified by combustion tests that the addition of 30 wt % EG(B) to LLDPE improved the limiting oxygen index (LOI) from 17.6% to 30.2%, and the vertical burning level of UL-94 standard reached V-0 level. Whereas, the LOI of the same amount referenced EG was only 25.1%, the UL-94 level only reached V-1. Moreover, the synergistic effect between EG(B) and ammonium polyphosphate (II) (APP) improved the LOI of 70LLDPE/10APP/20EG(B) composite to 33.0%, and the UL-94 level to V-0. The synergistic efficiency was attributed to the formation of continuous and compact residual char.