In this research, low-melting phosphate (LMP) glasses (G1, G2, G3) were employed as green flame-retardant synergist additives in epoxy (EP)/ammonium polyphosphate (APP) composite. To investigate their performance as flame-retardant additives, comprehensive fire tests and characterizations were carried out, including limiting oxygen index test, UL-94 vertical burning test, cone calorimeter test, real-time temperature detection, X-ray microtomography, and char residue strength analysis. A prominent improvement of flame-retardant properties was observed in EP composites containing LMP glasses. EP/9%APP/1%G1 reached V-0 rating in UL-94 test and showed a decreased peak heat release rate by around 40% over EP/10%APP in the cone calorimeter test. It found that LMP glasses mainly showed the physical barrier effect in the condensed phase flame retardancy. Interestingly, via simulating different combustion temperatures, LMP glasses reveal different flame-retardant performance. The mechanism was further elucidated as: A match between the LMP glass phase-transition temperature range and the combustion temperature contributed to the optimized flame-retardant performances. This result provided enlightenment for the design and application of phase-transition additives toward improving the flame-retardant properties in polymer composites.