To explore the effects of tanshinone IIA on the activity of aquaporin-5 (AQP5) in human alveolar epithelial cells (A549) after seawater exposure and its possible mechanism. Routinely cultured A549 cells were divided into different groups according to different content of seawater: blank control group, 15%, 25%, 50%, 75%, 100% seawater groups; they were divided into different groups according to the duration of exposure to 25% seawater: blank control group, 1, 4, 8 hours groups; they were also divided into different groups according to concentration of tanshinone IIA and exposed to seawater for 4 hours: blank control group, 25% seawater group, 25, 50, 75, 100 μg/ml tanshinone IIA intervention groups. The expressions of AQP5 were respectively assayed by Western blotting and immunohistochemistry. The results of Western blotting showed that the expressions of AQP5 were remarkably higher at 8 hours of exposure to seawater in 25% and 50% seawater groups than those in blank control group (1.053±0.231, 1.116±0.316 vs. 0.101±0.081, both P<0.05); the expression of AQP5 in 1-hour group showed a slight increase compared with blank control group (0.306±0.125 vs. 0.288±0.098, P>0.05), that in 4-hour group was increased significantly (1.423±0.377, P<0.01), and in 8-hour group (1.507±0.461) it was slightly higher than that in 4-hour group without statistical significance. The AQP5 expression was significantly lower in tanshinone IIA 25 μg/ml and 50 μg/ml intervention groups than that in 25% seawater group (0.580±0.186, 0.499±0.172 vs. 1.013±0.287, both P<0.05). Immuno-histochemistry showed that the expression of AQP5 was markedly up-regulated after A549 cells were stimulated with 25% seawater for 4 hours as compared with blank control group (7.21±0.78 vs. 0.41±0.07, P<0.01), but intervention of tanshinone IIA significantly inhibited the up-regulation of AQP5 expression (3.02±0.23) induced by 25% seawater (P<0.05). The experimental results showed that tanshinone IIA is innocuous to A549 at a dosage of 25 μg/ml, and it can decrease the overexpression of AQP5 induced by seawater.