Abstract: Using meso-scale data at the surface, the characteristics of a meso-β scale system causing a heavy rainstorm in Beijing area is studied. It is discoveyed that there is a dry line with the hot, dry air and cool, moisture air at its two sides respectively. The hot and dry air layer extends downward from the top of boundry layer, and is enhanced by downglide motion at the lee side of mountain. The increase of temperature and decrease of moisture become more drastic in that situation. The formation and structure of this system is different from the so called "dew-point front" commonly seen in China, even though they are all characterized by a zone of moisture discontinuity. However, it seems similar to the dry line occurring at the east side of the Rocky Mountains. The evolutions of the pressure, temperature and dew point depression during the whole life cycle of the dry line are given hourly. And the features of the stratification and radar echoes are also illustrated.