With the data published on Surface Meteorological Monthly Bulletin and Surface Chart during 1971-1996, the temporal variation of sand-dust storms invading Beijing is analyzed. Furthermore, the movement routines and source locations of sand-dust storms are determined in assistance with GIS. The results show: Sand-dust storms in Beijing occurred frequently in spring and early summer, especially in April. According to their source locations, the sand-dust storms are classified into two types, i. e. the internal and external-source sand-dust storm. For the external-source sand-dust storms, they always intrude Beijing from two major regions, one is the Wulanchabumeng, Erlianhaote and Abagaqi(western part of Xilin-guolemeng) of Inner Mongolia, the another is the Sino-Mongolia boundary from eastern part of Hami, to Alashanmeng. The source locations of internal-source sand-dust storms are mainly in Tenggeli desert and its surrounding areas.