The NCAR/NCEP re-analysis data and the daily precipitation data in July—August,1998 from 94 surface stations in the Songhuajiang and Nenjiang area were used to analyze vapor transport,which causes a severe flood of 1998.The analysis considers that the meridional transport of abundant vapor is the key.The water source comes from the low latitude south of the subtropical high over the west Pacific and the Indian Ocean.The strength of south jet,position of the subtropical high over the west Pacific and so on, are closely related to summer monsoon, affecting vapor transport.