A heavy rainstorm happened in North China during the period from 0000 UTC 16th to 0000 UTC 17th August 2005,and the strongest rainstorm occurred in Tianjin.By use of surface observation data and NCEP/NCAR reanalysis data and results of the mesoscale numerical model MM5V3.7,the circulation features and heavy rain in North China during 16th to 17th August 2005 were diagnosed and analyzed.The results show that the dry cold air intrusion had important impact on the rainstorm.The dry air in the upper troposphere intruded into the lower level was in favor of the development and enhancement of the meso-scale cyclone.