The gale process which happened in Laizhou Gulf on April 27, 2010 was analyzed diagnostically by using the NCEP data. This study shows that the explosive extratropical cyclone strengthened the frontogenesis behind the cyclonic cold front, thus increased the pressure tendency gradient which was the main reason for this gale process. By analyzing the synopsis environmental field, the temperature advection, the vorticity advection, the high level jet stream and the anomaly of the high level potential vorticity diagnostically, it is concluded that the strong barocline and the accompanied cold and warm advections made the high level trough develop. In the initial stage the vorticity advection difference between the high and low levels played an important role in the surface extratropical cyclone development. Then the cyclone developed further because of the divergence caused by the ageostrophic components to the left of the jet stream in front of the trough. The downward propagation of the potential vorticity on top of the troposphere triggered the surface cyclonic current and thus the cyclone developed explosively.