Based on the observational data, upper air and ground surface synoptic field during 2014~2015, and NCEP/NCAR reanalysis data in December 2015, combined with WRF-CMAQ numerical model, the characteristics and the cause of the air pollution process in Dalian were analyzed. The results showed that there were 145 pollution days, accounting for 20%, and 35pollution processes in Dalian from 2014 to 2015. The upper air circulations during air pollutions were mainly northwest air flow field after the trough (accounting for 63%), and the southwest air flow field before the trough (accounting for 21%). The ground surface synoptic field were mainly in two types, which are uniform pressure field with weak wind (accounting for 68%) and intensive isobars with high-speed wind. The atmospheric stratification was stable, with weak average horizontal wind during 5air pollution processes in December 2015 in Dalian, which suppressed the dispersion of pollutant in the vertical and horizontal directions, leading to the deterioration of air quality. Results of the numerical model simulation showed that nitrate, ammonium and sulfate are the main components of aerosols during the pollution process. And the nitrate provided the first contribution and the most obvious increase during the pollution processes, which means the vehicle emission and coal-fired emission already had important impacts on the formation of urban pollution in Dalian.