A scheme for coherent noise suppression by an off-axis illumination system was proposed, and an illumination system with the ability to suppress coherent noise was designed. The illumination system was simulated by the non-sequential mode of Zemax to obtain an off-axis annular light spot with a thickness of 90 μm. To verify the ability of this illumination system to suppress coherent noise, it was used as the light source of a 19.05 mm (3/4 inch) Fizeau interferometer. A comparative experiment was carried out with a light source wavelength of 632 nm, and scratches were set with the circular noise spots size of 120 μm~500 μm and the size of 100 μm×350 μm. Error analysis of this illumination system was performed to optimize the imaging quality of the illumination system and adjust the parameters of the interferometric system. Finally, the contrast of the interference fringe was improved to 97.60% with illumination ring thickness of 42 μm and interference cavity length of 15 mm.