The invention discloses a rare earth halide mixed scintillation crystal and a preparation method thereof. The rare earth halide mixed scintillation crystal has a chemical formula of Cs2Li(Y1-XLax)1-yCeyCl6, where x is greater than 0 and less than or equal to 0.4 and y is greater than or equal to 0.0001 and less than or equal to 0.1. The rare earth halide mixed scintillation crystal is prepared by a Bridgman-Stockbarge method and can be used in the fields of neutron detection, gamma ray detection or scintillation detection. The rare earth halide mixed scintillation crystal has single cell parameters changing along with change of x and y values. Through simultaneous occupation of La<3+> and Y<3+> ions on rare earth ion lattice sites, the rare earth halide mixed scintillation crystal has growth easiness of CLYC scintillation crystals and a high photo yield and good energy resolution of the CLYC scintillation crystals.