A cyst in the posterior knee joint is considered to be a popliteal cyst, which is most likely to be a benign lesion, although other cystic masses are not excluded. If the popliteal cyst is relatively small, does not produce obvious compression symptoms, has no effect on blood vessels or nerves, and does not affect normal knee flexion, it can be followed up and observed. If the popliteal cyst itself is large, causing compression of important blood vessels and nerves and affecting normal knee flexion, surgery may be considered. If it is a small popliteal cyst that increases significantly during observation, surgical treatment also needs to be considered. Routine postoperative pathologic examination of popliteal cysts to exclude other lesions and avoiding excessive knee flexion in general can help prevent recurrence.