The present invention relates to the field of machine vision measurement and relates to an epi-illumination light source having an adjustable diameter and an image measuring instrument. The epi-illumination light source (100) comprises an opening/closing transmission assembly, a drive assembly, and lamp panel groups. Each lamp panel group comprises a plurality of independent lamp panels (1) disposed on a same upper horizontal plane; each lamp panel group is connected to the opening/closing transmission assembly, and is driven by the drive assembly to move away from or approach the center of the light source so as to realize the adjustment of the diameter of the light source. The image measuring instrument having a large-diameter lens is used, and the diameter of the light source is adjustable. When using a low-magnification lens, the diameter of the light source becomes larger, there is no gap between the lamp panels, and the field of view is not blocked, ensuring uniform illumination of the entire field of view; when using a high-magnification lens, the diameter of the light source becomes smaller, the lamp panels overlap, and light is incident at a small angle to an object under test, meeting feature extraction requirements of the high-magnification lens.