Disclosed are a silicon nitride ceramic sintered body and a preparation method thereof. The silicon nitride ceramic sintered body has a content of a silicon nitride crystalline phase of not less than 98 wt %, a relative density of not less than 99%, a porosity of not larger than 1%, a grain boundary phase including Li, O, N, and Si elements, and a total content of C, F, Al, Mg, K, Ca, Na and rare-earth metals elements of less than 0.1 wt %.