The heaviest fog in winter of 2009 in Sichuan province is simulated by the mesoscale model WRF,and the results show that both the simulated fog regions and relative humidity are in accordance with the observed data.The parameterization of Vis-RH is acquired by deterministic and probabilistic methods based on the data of visibility and relative humidity observed in Chengdu Shuangliu Airport,then the simulated relative humidity is used to calculate the visibility.The study shows that the visibility values calculated by the Vis-RH parameterization with 5% probability is most accurate,and this method can provide reference for the visibility forecast in the future.