Abstract: This article analyzed the mesoscale mechanism of one heavy fog process that happened on the November 13~14 2009 in the northern part of Henan province, based on the MICAPS high altitude, ground observation data and NCEP 1°×1°re-analyzed data. The author found that the mesoscale convergence on the ground was synchronous to the formation of this heavy fog process what in the time concomitance relations. Three reasons lead to this heavy fog process had varied, which are the changed of water vapor flux, the temperature thermal advection and the surface radiation cooled. Which physically transformed radical fog into advection fog. This analysis indicate that, before 8:00 on November 13, the snowfall brought the aqueous vapor and the sunny frigid night weather condition caused the surface radiation cooling which formed the radical fog. After 14:00 on November 13, the increased of the water-vapor flux, the warm advection at the same time and the ground continually decreased temperature, that caused the warm air and wet air on the frigid surface cooled down what made the radical fog physically transformed into advection fog.