Abstract: The rapid development of copper-clad laminates has raised higher requirements for the flame retardant and dielectric properties of the matrix materials. A Schiff base derived flame retardant (PBDDM) containing P, N, and B was successfully synthesized by modifying epoxy resin (EP) with low molecular weight polyphenylene oxide (PPO) and using 4,4-diaminodiphenylmethane, 4-formylphenylboronic acid, and diphenylphosphine oxide. Its effects on the combustion characteristics, fire safety, and dielectric properties of EP-PPO materials were studied, and its flame retardant mechanism was analyzed. The results show that PBDDM/(EP-PPO)-4% achieves a UL-94 V-0 rating and a limiting oxygen index (LOI) of 27.3%. Moreover, PBDDM/(EP-PPO)-6% exhibits the lowest fire growth rate index (FIGRA) and the highest fire performance index (FPI); compared with PBDDM/(EP-PPO)-0%, FIGRA is reduced by 60.0% and FPI is increased by 59.8%. This is the result of the synergistic flame retardant effect of P/N/B in both gas and condensed phases. In terms of dielectric properties, PBDDM forms a cross-linked network structure with EP-PPO, increasing the free volume fraction and restricting the movement of charge carriers, thereby improving the dielectric properties. Compared to PBDDM/(EP-PPO)-0%, the dielectric constant (D_k) and dielectric loss (D_f) of PBDDM/(EP-PPO)-6% are reduced by 31.1% and 68.3%, respectively. This provides a feasible strategy for designing and preparing high-performance EP-PPO materials that combine flame retardancy and dielectric properties.