Blackening treatment method for improving conductivity of lithium niobate or lithium tantalate wafer

The invention discloses a blackening treatment method for improving the conductivity of a lithium niobate or lithium tantalate wafer, which comprises the following steps: mechanically ball-milling and mixing a reducing elementary substance material, lithium carbonate powder, plant fiber or plant starch and water to obtain a composite reducing agent; the preparation method comprises the following steps: coating the front and back surfaces of a pretreated lithium niobate wafer or lithium tantalate wafer with a composite reducing agent in a spraying or spin coating manner, drying, alternately stacking the wafers in a container, transferring the container into a heat treatment furnace, vacuumizing the heat treatment furnace, adjusting the temperature in the heat treatment furnace, and carrying out heat treatment under a nitrogen atmosphere, so as to obtain the lithium tantalate wafer or lithium tantalate wafer. According to the technical scheme, yellow-black wafers/grey wafers with different reduction degrees can