To investigate the relation between air pollution exposure and preterm birth in Shanghai,China.Methods We examined the effect of ambient air pollution on preterm birth using time-series approach in Shanghai in 2004. This method can eliminate potential confounding by individual risk factors that do not change over a short period of time. Daily numbers of preterm births were obtained from the live birth database maintained by Shanghai Municipal Center of Disease Control and Prevention. We used the generalized additive model (GAM) with penalized splines to analyze the relation between preterm birth,air pollution,and covariates.Results We observed a significant effect of outdoor air pollution only with 8-week exposure before preterm births. An increase of 10 μg/m3 of 8-week average PM10,SO2,NO2,and O3 corresponded to 4.42% (95%CI 1.60%,7.25%),11.89% (95%CI 6.69%,17.09%),5.43% (95%CI 1.78%,9.08%),and 4.63% (95%CI 0.35%,8.91%)increase of preterm birth. We did not find any significant acute effect of outdoor air pollution on preterm birth in the week before birth.Conclusion Ambient air pollution may contribute to the risk of preterm birth in Shanghai. Our analyses also strengthen the rationale for further limiting air pollution level in the city.