<p>A modified silicone oil containing nitrogen, phosphorus, and sulfur was synthesized by the reaction of n-butyl thiophosphoric triamine (NBPT) and epoxidized silicone oil (ETSO), and named NBSO. The successful synthesis of NBSO was confirmed by Fourier infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). The flame retardant and smoke suppression properties of NBSO were investigated by open flame ignition method and cone calorimetry test (CCT). The results showed when the molar ratio of NBPT to ETSO was 1:1, the flame height of the modified silicone oil was significantly reduced, and compared with ETSO, the total smoke release (TSR) and the total heat release (THR) of NBSO was reduced by 67.6% and 68%, respectively. Thermogravimetric analysis (TGA), infrared thermogravimetric coupling (TG-IR), thermal field emission scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques were used to analyze the mechanism of flame retardant and smoke inhibition. The results indicate that compared with ETSO. NBSO has a slower thermal decomposition rate and releases less flammable gas.In the condense phase, nitrogen, phosphorus, silicon, and sulfur could improve the carbon forming ability through synergistic effect. At the same time, a silicon oxide layer was formed on the outer surface of the carbon layer, which could increase the flame retardation and smoke suppressing effect.</p>