Based on the routine data and Doppler radar echo etc, a continuous heavy rain event in Weihai, Shandong Province from 4 to 6 August 2004 is diagnosed. The meso-β-scale heavy rain in the evening on August 5 is analyzed on emphases. It shows that the event occurred in the area of high energy accumulation at the edge of the west Pacific subtropical high. And the main reason of the heavy rainfall in the first period is the cold air invasion. Surface convergence is the primary trigger mechanism to lead the heavy rainfall again. The rich moisture transmission and the concentrated CAPE (convection available potential energy) provided the physics mechanism for the occurring and development of mesoscale heavy rainfall system.