The Einstein probe is a mission for time-domain astronomy. The follow-up X-ray telescope is a scientific payload on the EP. FXT is equipped with a nested gold-plated nickel Wolter-I X-ray telescope, which has an advantage in terms of angular resolution, a critical performance parameter for the Wolter-I X-ray telescope. The Wolter-I X-ray focusing mirror contributes the most angular resolution of the telescope. The angular resolution is typically represented in terms of the half-power diameter. Because its optical path is the same as X-rays, visible light can be used to measure the angular resolution performance of focusing mirrors instead of X-rays. However, this also results in the effect of visible light diffraction. This work evaluates the diffraction effects in a visible light test and derives an equation for determining the diffraction distribution of an annular aperture with a very high obstruction ratio based on the Fraunhofer diffraction theory. The angular resolution is measured as 32.02"±0.44" in the visible light test using the 18th EP-FXT mirror shell under a 473 nm parallel laser, and the contribution of the visible light diffraction is calculated to be 20.15". The value for the mirror shell is 24.90"±1.61". Compared to the 100 m vacuum X-ray calibration facility measurements of 25.10"±1.55" at the Institute of High Energy Physics, Chinese Academy of Science, these results are consistent within the margin of error.