A heavy pollution occurred in Beijing during 7th-10th April 2006. The synoptic situation, meteorological element, flow field and atmospheric stability of the event are analyzed. The results show that the same sand dust affected Be ijing twice and caused continues heavy weather pollution. For the first time, th e sand dust of inner Mongolia came down along with the north-west flow and reach ed Beijing. For the second time, based on “refluence" weather background, along w ith the easterly wind, the sand dust which had moved to downstream returned Beiji ng. The heavy weather pollution is a common action result of sand dust and ca lming weather condition.