Objective To discuss the relationship between TCM constitutional types and the levels of transforming growth factor beta 1 (TGF-β1) of patients with knee osteoarthritis (KOA). Methods A total of 161 patients with KOA as a case group filled out questionnaires about 9 TCM constitution types, and 50 cases of unrelated healthy volunteers were selected randomly as control group. The serum samples of two groups were collected. The levels of TGF-β1 were detected and compared by double antibody sandwich ELISA. Results Compared with the control group, the level of TGF-β1 in the case group decreased, with statistical significance (P<0.05). Compared with the control group, the levels of TGF-β1 significantly decreased in the case group of qi deficiency type, with statistical significance (P<0.05). There were no types of special intrinsic and blood stasis. Compared with the control group, the levels of TGF-β1 in the other six types of patients with KOA had no statistical significance (P>0.05). Conclusion The incidence of KOA with qi deficiency type may be related to the decrease of TGF-β1 level. The decreasing level of TGF-β1 may be one of the mechanisms of molecular biology that qi deficiency was linked to KOA.