Objective To study effects of paraquat on proliferation and viscoelasticity of human embryonic lung fibroblasts (MRC-5), and to discuss mechanism of MRC-5 damage at initial stage and pulmonary fibrosis later after paraquat intoxication. Methods MRC-5 cells were treated by culture medium with different concentration of paraquat (50, 100, 200 mg/L, respectively) for 12 hours, when the paraquat culture medium was replaced by normal culture medium. At 48th hours, MRC-5 cells were collected, examined and analyzed by flow cytometry for indicating the proliferation, and micropipette aspiration technique was used to investigate viscoelasticity of the cells. Results After treated by paraquat with different concentration, proliferation index (PI) of MRC-5 cells were significantly reduced, as compared with the control group (P<0.05), and PI was reduced with the increase of paraquat concentration; viscoelastic parameters of MRC-5 cells were significantly decreased as compared with the normal cells (P<0.05); viscoelastic parameters of MRC-5 cells in 100 mg/L group and 200 mg/L group were smaller than those in 50 mg/L group (P<0.05), and there was no significant difference between 100 mg/L group and 200 mg/L group (P>0.05). Conclusions MRC-5 cells were damaged at initial stage of paraquat intoxication, with PI and viscoelastic parameters reducing. Decompensated repair after paraquat intoxication is an important reason leading to pulmonary fibrosis, which provides a new thought in clinical treatment.