An extremely heavy rainfall event occurred in Guangzhou City on 7 May 2017. The record-breaking heavy precipitation caused severe property damage. To investigate the reasons/mechanisms responsible for the severe rainfall, a detailed observation analysis was performed in this study, based on the dataset collected by ground-based Doppler radars, two-dimensional video disdrometer (2DVD), microwave radio- meter and wind-profiling radar. The research results show that the ambient conditions prior to this event are characterized by small value of convective inhibition (CIN), low lifting condensation level (LCL), moderate convective available potential energy (CAPE), deep warm layer, and moist southerly flow. From midnight to dawn of 7 May 2017, initial convection was generated mainly by the terrain-blocked southerly flow, leading to the formation of quasi-stationary rainbands. From sunrise to early morning, new convective cells were repeatedly triggered along the precipitation-induced outflows boundary (i.e., back building process) and were continuously propagated backward (i.e., “echo training”). The observaiton of 2DVD further suggests that the surface heavy precipitation was composed of a high concentration of small raindrops and a few large raindrops. This microphysical information indicates that the high-precipitation-efficiency warm rain process was the main microphysical mechnism responsible for the heavy rainfall. This argument is supported with the low-centroid cumulonimbus structures observed by Doppler radar. It suggests that the continuously intensifying southerly flow played an important role in sustaining the convective system and producing the local heavy rainfall.