After a brief introduction of Shanghai mesoscale numerical weather forecast system, a real time forecast result of a heavy rain in East China during 29/05/2002 to 30/05/2002 is analyzed with output of this forecast system. The macro scale stream field and simulation result are analyzed and the main causes of the event are discussed. Furthermore, the predictands in correlative single stations are discussed. The results proved that the Shanghai mesoscale numerical weather forecast system has the ability at a certain extent in operation, and is worth of reference to forecasters.