The bandpass filter has been utilized to analysis two cases of squall line event in North China. The realistic wind field has been successfully seperated into a slowly variable, more stable macroscale environment field and an obviously movable meso-scale disturbances field. In general, there are several meso-a scale(250-2500km) disturbances emerged into the disturbances field, some of them is deep and moving steadily. The squall line systems occurred in a region of strong air flow associated with one of these disturbances, which is larger and formed earlier and persisted quite longer than the life cycle of the squall lines.