This paper conducts a psychological analysis on the content related to “forgetting” in the text of Liezi. The results show that the “forgetting” in Liezi encompasses five levels of the cultivation of the mind and nature, namely: the daily forgetting of “being attached to something in the mind without being aware of it oneself ”; the forgetting in handling affairs of “forgetting the coarse and the external”; the forgetting in cultivating virtue of “the superiors forgetting while the inferiors not rebelling”; the unintentional forgetting of “forgetting due to illness”; and the forgetting in inaction of “forgetting spontaneously when awakened” and “knowing but forgetting emotions”. These five levels progress layer by layer and are integrated as a whole, jointly constituting the thought of the cultivation of the mind and nature regarding “forgetting” in Liezi. Uncovering this traditional Taoist thought has certain enlightening effects on contemporary psychological education and psychotherapy.