OBJECTIVE： To study the effects of Thymalfasin for injection on the apoptosis of human lung cancer A549 cells. METHODS： After treated with 0 （blank control）， 25， 50， 100， 200 and 400 mg/L Thymalfasin for injection for 24， 48 and 72 h， the cell proliferation inhibitory rate was analyzed with MTT and calculated. After treated with 0 （blank control）， 50 and 100 mg/L Thymalfasin for injection for 48 h， cell apoptosis was detected by flow cytometry， and the expression of Caspase-3， Bcl-2 and Bax and the phosphorylation level of Akt were deteced by Western blot. RESULTS： Compared with blank control group， proliferation inhibitory rate of A549 cells increased after treated with Thymalfasin for injection， in concentration and time-dependent manner （P＜0.05）. The apoptotic rate of A549 cells increased after treated with Thymalfasin for injection 50， 100 mg/L for 48 h （P＜0.05）. The expression of Caspase-3 increased while the Bcl-2/Bax and phosphorylation level of Akt decreased in A549 cells after treated with Thymalfasin for injection 100 mg/L （P＜0.05）. CONCLUSIONS： Thymalfasin for injection can inhibit the proliferation of A549 cells by activating Caspase-3， decreasing Bcl-2/Bax ratio， inhibiting Akt signal pathway and induce the apoptosis of A549 cells.