An effective flame retardant and smoke suppression oligomer, poly(melamine-ethoxyphosphinyl-diisocyanate) (PMPC), was successfully synthesized. The chemical structure was characterized by Fourier transform infrared (FTIR), H-1 nuclear magnetic resonance (H-1 NMR), and P-31 nuclear magnetic resonance (P-31 NMR) spectroscopies. PMPC was used alone as the flame retardant for epoxy resins (EP). The results showed that PMPC endowed EP with good flame retardancy. The limiting oxygen index (LOI) value of a EP/PMPC system containing 20 wt % PMPC increased to 28.0%, and can achieve a UL-94 V-0 rating. The cone calorimeter data showed that the heat release rate (HRR) and total heat release (THR) were considerably reduced with the addition of PMPC, and the smoke production rate (SPR), total smoke production (TSP), and carbon monoxide production (COP) were also reduced. Moreover, the thermal degradation behavior of PMPC and the flame-retardant mode of the EP systems were also investigated.