The application discloses a preparation method of monocrystal uranium dioxide nuclear fuel pellets, comprising: granulating and pelleting UO2 powder to obtain UO2 pellets; then coating surfaces of the UO2 pellets with monocrystal growth additive micro powder to form core-shell structure particles; and activated-sintering the core-shell structure particles at high temperature, liquefying the monocrystal growth additive on the surface of the core-shell structure particle at high temperature and then diffusing into UO2 pellets, dissolving the UO3 in the liquid monocrystal growth additive, and recrystallizing the UO2 to form the monocrystal UO2 nuclear fuel pellets.