Objective:To study the anti－proliferative effect of vitamin D on the proliferation of airway smooth muscle cells(ASMCs) and prove the related mechanism of cyclin D1 and vitamin D in asthmatic rat. Methods:The ASMCs in male Sprague Dawley(SD) rats of control group and asthma group were cultured from primary generation. Cells between passage 3 and 5 were used in the study and were divided into control group,asthma group,asthma + 1 × 10-6 mol/L vitamin D group,asthma + 1 × 10-7 mol/L vitamin D group and asthma + 1 × 10-8 mol/L vitamin D group. The proliferation of ASMCs among each group was examined by MTT colorimetric assay and cell cycle analysis. The expression of cyclin D1 was detected by reverse transcriptase polymerase chain reaction(RT－PCR)and Western blot,respectively. Results:①The percentage of S(8.31 ± 0.42)% and G2/M(11.21 ± 1.43)% phase,the absorbance(A)(0.78 ± 0.042) value in ASMCs from asthma group were significantly increased compared with other groups;but the percentage of G0/G1[(80.01 ± 2.55)%] phase were significantly decreased,and compared to those(G0/G1,S and G2/M phase,A value) in ASMCs from control group[(90.15 ± 3.02)%,(5.41 ± 0.62)%,(4.38 ± 0.56)%,0.22 ± 0.04] and asthma + 1 × 10-6 mol/L vitamin D group[(89.08 ± 2.81)%,(5.58 ± 0.92)%,(4.61 ± 1.12)% and 0.38 ± 0.02,P < 0.001]. The percentage of G0/G1,S and G2/M phas,A value in ASMCs from asthma + 1 × 10-7 mol/L vitamin D group and asthma + 1 × 10-8 mol/L vitamin D group were compared with asthma group respectively(P < 0.01);②There were significant differences in the ratios of A value of cys D1 mRNA and expression of cyclin D1 protein in ASMCs from asthma group compared with control group and asthma + 1 × 10-6 mol/L vitamin D group respectively (P < 0.001) ;③And in the proliferation of ASMCs and the cyclin D1 mRNA and protein expression among different concentration of vitamin D groups,there were significant differences between the high and the low concentration(P < 0.001). Conclusion:ASMCs in asthmatic rats proliferate significantly and the expression of cyclin D1 increases obviously. Vitamin D can suppress the proliferation of ASMCs in asthmatic rats via regulating cyclin D1 expression.