The heavy fog-haze episode occurred in Beijing-Tianjin-Hebei region during December 5-10, 2015 was simulated using GRAPES_CUACE (Global/Regional Assimilation and PrEdiction System coupled with the China Meteorological Administration Unified Atmospheric Chemistry Environment). Characteristics of wind speeds at the surface, PBLH (Planetary boundary layer height), relative humidity and PM_2.5 concentration as well as their impacts on visibility were analyzed in detail. The results show that the diurnal variation of visibility and PM_2.5 mass concentration in Beijing-Tianjin-Hebei region could be well simulated while the low visibility during the heavy fog-haze pollution was over estimated. The average wind speed was lower than 2 m s^-1 and the average PBLH was lower than 600 m in most areas during the fog-haze episode. The overestimation of the low visibility by the model are due to the following reasons according to the simulation analysis. First, the maximum concentration of PM_2.5 during the heavy fog haze episode was underestimated; second, the simulated relative humidity was lower than observations, which affected the hygroscopic growth process of soluble aerosols and the concentration of aerosols, leading to changes in aerosol extinction; and the current parametric formula doesn't take into consideration the direct extinction of fog-drops. These results indicate that the simulation and prediction of surface relative humidity have important impacts on accurate simulation of low visibility during heavy fog-haze episodes.