Objective : To explore season-specific association between ambient PM2.5 and outpatient visits for asthma in the urban areas of Ningbo. Methods : Based on the platform of population health information,we collected daily outpatient visits for asthma in four general hospitals in the urban areas of Ningbo from 2014 to 2016. We also collected data of meteorological indicators and air pollutants during the same period from local weather bureau and environmental monitoring center. Generalized additive model（GAM）was applied to examining the relationship and lag effects between ambient PM2.5 and outpatient visits for asthma in different seasons. Results : A total of 45 184 outpatients with asthma were reported from 2014 to 2016,with an average of 41.23 cases per day. The annual average concentration of PM2.5 in Ningbo was（43.12± 26.40）μg/m3 ,with the peak as（65.27± 34.51）μg/m3 in winter and the though as（27.34± 12.70）μg/m3 in summer. A 10 μg/m3 increase of PM2.5 were associated with 1.14%（95%CI：0.48%-1.81%）,2.40%（95%CI：0.22%-4.63%）and 1.37%（95%CI：0.28%-2.48%）increase of outpatient volume for asthma on the day in the whole year,summer and winter,respectively. In summer,males were at higher risk compared to females（P< 0.05）;in winter,people under 65 years old were at higher risk（P< 0.05）. Including NO2,the effect of PM2.5 on the excess risk of outpatient visits for asthma in summer was enhanced in two-pollutant models（P< 0.05）. Conclusion PM2.5 pollution might increase the outpatient volume for asthma in summer and winter. NO2 might have synergistic effect with PM2.5 on asthma.