The lateral growth of single-crystal diamond on a semi-open substrate holder was realized in this paper by microwave plasma chemical vapor deposition (MPCVD). The protrusion height (Δh) of the seed crystal above the substrate holder was adjusted to regulate the radicals distribution. The influence of protrusion height on the epitaxial lateral growth of single-crystal diamond was analyzed by combining optical emission spectrum with the Fourier transform infrared spectrum, Raman spectrum, white light interferometry results and optical morphology characterization outcomes. The analysis show that the relative concentration of C₂ (516.08 nm) radicals in the central region (from -2 mm to 2 mm) of the plasma increased with increasing protrusion height. When the protrusion height was increased to 0.6 mm, the concentration of carbon-related radicals in the central region was relatively high, resulting in a vertical growth rate slightly faster than that in the surrounding area. The growth rate difference is favorable for the growth plane to automatically develop structures in an off-axis direction from the (100) crystal plane. The result of lateral growth of these structures is the formation of single-crystal diamonds without the polycrystalline diamond rim. These diamonds also have excellent infrared transmittance. The automatic development of off-axis structures on the top growth plane was the key to epitaxial lateral growth of single-crystal diamonds. However, the continued increase of protrusion height to 0.8 mm led to an unnecessarily high relative concentration of C₂ (516.08 nm) radicals in the central region and formation of pyramidal hillocks, which is the disadvantage of the epitaxial growth of high-quality single-crystal diamonds.