The invention provides a rhombus plug gauge processing method includes the following steps of preparing materials, selecting forge piece blanking which is intact in materials, and free from flaws and sand holes, carrying out lathe operator process, miller process, heat treatment process, sand blowing process, surface treatment process, and cutter grinding process, firstly grinding a D surface and a surface parallel to the D surface until required sizes are reached, then enabling a rhombus plug gauge to be clamped on a sine platform, enabling the grinded D surface and the surface parallel to the D surface to face the left side face and the right side face of the sine platform, wherein two non-grinded surfaces face the upper surface and the lower surface of the sine platform, calculating thickness of a feeler gauge according to angle of the rhombus, enabling the feeler gauge to be cushioned under the sine platform, enabling a grinding wheel to begin to carry out cutting feed along the track of the grinding wheel, firstly grinding an E surface, then turning the rhombus plug gauge to begin to grind a surface parallel to the E surface after the E surface is well grinded, carrying out fitter process, and carving marks. The rhombus plug gauge processing method reduces processing difficulty of the rhombus plug gauge, and ensures accuracy requirements of the rhombus plug gauge.