The invention discloses a spherical core-shell structured Al2O3/AlN thermal-conducting powder and a high-thermal-conductivity insulating silicone grease and preparation methods thereof. The method is implemented by taking spherical Al2O3 as a raw material through reacting the spherical Al2O3 with a carbon source for 8-12h in a N2 atmosphere and at a temperature of 1400-1650 DEG C so as to obtain the spherical core-shell structured Al2O3/AlN thermal-conducting powder, and after the powder is treated by using a coupling agent, mixing the obtained object with silicone oil. According to the invention, a thermal-conducting silicone grease is prepared by using the high filling and excellent thermal-conducting and insulating properties of the spherical core-shell structured Al2O3/AlN thermal-conducting powder, the coefficient of thermal conductivity is not less than 5.0 W/m.k, the volume resistivity is greater than 1.0*10<15>Omega. cm<-1>, and the viscosity is not greater than 100000 mPa. S (25 DEG C), so that the silicone grease can be kept in a paste state after being placed for a long time, and is oiltight, therefore, the silicone grease can meet the operating requirements of high-power electronic devices which need high heat dissipation and insulation properties; and the silicone grease is simple and efficient in operation and low in production cost.