This invention discloses a ceramic material of silicon nitride with high heat conductivity, and its preparation method. Due to the use of rare element fluoride YF3 is substituted for rare elements oxides as agglutinant, and due to the adding of alkaline metal oxide MgO, to lower the molten temperature and viscosity of the glass phase produced during sintering, so to lower the sintering temperature to make the crystal boundary bing crystallization, to increase heat conductivity of this invention product being higher than that of prior art product by 14%, and bending strength by 9%. This invention has important value in commercial application.