In order to improve the planetary boundary layer (PBL) parameterization over complex underlying surface for better calculation of vertical turbulent transport of energy and substances, and verify the effect in numerical simulation over China, an improved MYNN (Mellor-Yamada-Nakanishi-Niino) PBL parameterization scheme is selected in the WRF model to simulate two heavy fog cases that happened in the Yellow Sea on March 17th, 2009 and in North China on December 4th, 2011, respectively. The new MYNN scheme is tuned based on the observational PBL data over different underlying surfaces. In this paper, we attempt to simulate the evolution of both the land and sea fog events, and explore the boundary layer structure and the impact of the modified MYNN scheme on these simulations. With reference to the FY-3 satellite imageries and sounding data, it is found that better simulation of the horizontal distribution of the vertically integrated cloud water can lead to better simulation of generation, movement and spatial distribution of the heavy fog over both the ocean and land areas. The vertical profiles of temperature and mixing ratio of cloud water illustrate more reasonable vertical structure of the fog and the stable stratification. In addition, the new scheme improves the low-level vertical distribution of potential temperature and the specific humidity in the fog area.