Nanocomposite PMN-PT-BT/Ag/MgO was prepared by sintering at $950{circ}C$ with addition of $AgNO_3$ and MgO sol to the PMN-PT-BT powder sinterable at $1200{circ}C$. The low-temperature-sinterable PMN-PT-BT/Ag powder prepared by the modified mixed oxide method was calcined at $600{circ}C$ for 1h and surface modified with the MgO sol of 0-10 wt% and then subjected to consolidation at $850-950{circ}C$ for 4h under a flowing oxygen. The nanocomposite PMN-PT-BT/Ag/MgO(0.5wt%) sintered at $950{circ}C$ showed the microstructure with grains of $1-3{mu}m$, the second phase of MgO of $0.1-0.3{mu}m$ by SEM and Ag of << $1{mu}m$ qualitatively by SIMS. It showed the sintered relative density of 99%, the room temperature dielectric constant of 17200, the dielectric loss of 2.1% and the specific resistivity of $5.46{ imes}10^{12}{Omega}{cdot}cm$. But the PMN-PT-BT/Ag/MgO(0 wt%) nanocomposite sintered at $950{circ}C$ showed a little better properties : the sintered relative density of 99.5%, the room temperature dielectric constant of 19500, the dielectric loss of 2.1% and the specific resistivity of $7.30{ imes}10^{12}{Omega}{cdot}cm$.