We describe the measurement of atmospheric enclosure seeing along a 120-m light path by use of a Shack–Hartmann wave-front sensor S-H WFS for the first time to our knowledge in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST outdoor active-optics experiment system, based on the differential image motion method and a S-H WFS. Seeing estimates that were gained with the S-H WFS were analyzed and found to be in close agreement with the actual seeing conditions, the estimates of refractive-index structure constant, and the thin-mirror active optics results, which usually include the shape sensing precision and the active correction precision of the experimental system. Finally, some countermeasures against poor seeing conditions were considered and adopted.