A heavy fog/haze event occurred in Beijing from 27 November to 1 December 2015 The process was synthetically analyzed based on the synoptic situation, radisonde data, the conventional and intensive observational data, the wind profile radar data, and the observational PM25 concentration. The results show that the deterioration of visibility has a close relationship with synoptic situation, the significantly increased pollutant particles, the increase of relative humidity, the high frequency of temperature inversion, small nearsurface wind speed, and nearsurface southerly transports. It is concluded: (1) It was long controlled by the weak pressure system on the ground over North China and the nearsurface wind speed was small during the fog/haze process and large parts of Beijing were controlled by the weak south and east wind which were not favorable to diffusion and transportation of pollutants. (2) The relative humidity increased apparently and sometimes close to saturation, due to the transferring of water vapor and pollutants by the south and east airstream and the melted snow. (3) The inversion of temperature in the boundary layer existed during the whole fog/haze process and it was not conducive to the vertical diffusion of pollutants.