The invention relates to a single crystal diamond and a preparation method thereof. The preparation method comprises the following steps of : S1, preparing a diamond nucleation layer on a substrate; S2, epitaxially growing a polycrystalline diamond layer with uniform crystal face orientation and uneven surface on the diamond nucleation layer; S3, epitaxially growing metallic iridium on the uneven surface of the polycrystalline diamond layer; S4, performing surface polishing treatment on the metal Iridium to expose the planes where the polycrystalline diamond layer and the metal Iridium are distributed in a staggered manner to form a composite substrate; S5, growing single crystal diamond on the composite substrate. The preparation process can form nucleus on the surface of polycrystalline diamond with homogeneous crystal plane orientation. The requirement of direct current bias voltage on the substrate in the nucleation stage in the traditional monocrystal diamond heteroepitaxy process is avoided, and the formed monocrystal diamond is uniform in crystal orientation and high in quality, so that the monocrystal diamond is compatible with the existing equipment, the process complexity is reduced, and the yield is improved.