On the basis of analysis of the satellite cloud picture,combined with routine meteorological information and the product of T106 mathematical forecast,the mechanic of a local torrential rain in Beijing on 23 July,1998 and the character of mesoscale structure about IR cloud atlas were analyzed.It is shown that the advantageous conditions are favorable to the birth and development of the mesoscale system including the favorable synoptic condition,the water vapor convergence and high instability energy.