High-energy photon source (HEPS) is a 6 GeV ultralow emittance storage ring light source to be built in Beijing, China. Both the horizontal and vertical beam sizes of the HEPS storage ring are below 10μm. It is a challenge to measure such a small beam size in both directions. To this end, measurement by a Kirkpatrick–Baez (KB) mirror imaging system was evaluated. A test KB system for the Shanghai Synchrotron Radiation Facility storage ring was designed and tested. Two crossed cylindrical mirrors were used to image the dipole source point. Both mirrors can be moved in and out so that the monitor is interchangeable with the original X-ray pinhole monitor. The aberration and point spread function, which would cause image blur, were evaluated. A beam-based calibration scheme was used by varying the beam size with different quadrupole settings and fitting them with the corresponding theoretical values. We updated the original X-ray camera with a new camera having a 5-μm-thick LuAG/Ce scintillator, and the imaging result shows greatly decreased image blur.