Frequent haze events have been happening in North China, and the severity of these events has attracted massive attention from both public and scientific community. Extremely high aerosol loadings in these haze events significantly influence visibility, human health and climate. We explored the causes of winter haze in North China from several aspects, including meteorology role, secondary aerosol formation role, regional transport and role, and aerosol feedbacks role. The winter haze events in the NCP are mainly caused by high emissions of air pollutants, stagnant winter weather (low wind speeds, strong temperature inversion), and secondary aerosol formation. Besides, non-local contributions are comparable to local contributions and aerosol feedbacks can aggravate haze pollution. Compared to observations, sulfate concentrations have been largely underestimated by current models, indicating some missing winter sulfate formation pathways in the model. Although there have been great advances in aerosol modeling, it is still challenging due to large uncertainties in emission inventory, physical and chemical parameterizations. We assimilated surface PM2.5 concentrations to constrain model predictions and reduce the uncertainties in estimates of health and climate impacts, which can provide important scientific and policy basis for haze pollution control.