The cure behavior of epoxy resin E51–melamine phosphate (MP)–methyltetrahydrophthalic anhydride (MeTHPA)–2-ethyl-4-methyl-imidazole (2,4-EMI) halogen-free fire retardant composite at varying temperatures and MP loading is investigated by the dynamic torsional vibration method (DTVM). The dynamic parameters, such as gelation time tg, apparent activation energy Ea, and curing rate k are estimated as well. The theoretical prediction is in good agreement with the experimental results obtained by the DTVM. The results show that the rate of cure reactions depends distinctly on the cure temperature. At low MP loadings, the tg for the systems does not change obviously, while at higher MP loadings, it has a remarkable increase. Furthermore, the Flory's gelation theory and the Avrami equation are used to describe the cure behavior of the halogen-free fire retardant composite system.