Using the NCEP 1°×1° grid data, Doppler radar data, and products from the SWAN system, this paper analyzes the largescale heavy precipitation process in Guangxi at 20:00 BT 2 to 20:00 BT 4 July 2009 with a comprehensive diagnostic method. The results show that: The cold air is very weak so as to penetrate into Guangxi, then the southwestern warm low can maintain for a long time, this is one of the main causes for the precipitation lasting for long time. Meanwhile, the surface convergence line is stable, which is the trigger mechanism of strong convection development in northern Guangxi, and is also the main factor for the generation of ‘train effect’. Base reflectivity of Liuzhou Doppler radar showed that the “train effect” made heavy rainfalls occurring in areas east of Hechi and north of Liuzhou during 20:00 BT 2 July to 08:00 BT 3 July 2009, and storm center is reflected as strong convergence in the radial velocity. Besides, the SWAN products were tested and analyzed, the Cotrec winds have a good indication to the echo moving; TITAN techniques have some forecasting capability to the movement and intensity of strong echo; and the onehour rainfall forecast is generally similar to the live rainfall.