Abstract: Guangzhou was hit by an unexpected local torrential rain on 7 May 2017, which occurred without any obvious synoptic weather system on the edge of the subtropical high. The torrential rain that mainly occurred in complex terrain area showed obvious mesoscale characteristics including large hourly local rainfall intensity and long duration. The triggering and maintenance mechanisms have attracted great attention of researchers and forecasters. Mesoscale characteristics and mechanism analysis for the triggering and maintenance of the mesoscale convective systems (MCS) were studied and discussed based on high spatial and temporal resolution data such as observations at 5 min intervals collected at automatic meteorological stations, 1 min rainfall observations, and observations of wind profile radar and Satellite Himawari 8, etc. Results show that easterly winds on the southern side of a high-pressure system in the mid-latitude that was moving towards the sea were accompanied with the development and intensification of a shear line to the west of Guangzhou, leading to a favorable condition for the convergence of southerly winds that were enhanced by the trumpet-shaped topography in the Pearl River Delta. Dense isotherms in northern part of Guangzhou were caused by urban heat island effect and the underlying surface cooling. Mountain winds and southerly winds slowed the movement of the convergence line, which is helpful for the formation of tropical cloud clusters. Topographic blocking and lifting along with large temperature gradient further strengthened the ascending motion. The southerly wind velocity fluctuation enabled the cloud clusters to move towards the mountain quickly and finally resulted in convection outburst. Weak cold pool outflow brought by untypically deep convective storm under the condition of subtropical high made MCS to propagate continuously, which enabled the strong rain echo to enlarge quickly. The center of convection remained near Zengcheng due to effects of the small-scale terrain, finally leading to extreme hourly rainfall. The moving direction of the MCS was almost opposite to its propagating direction, resulting in slow movement of the MCS. Meanwhile, backward prorogation gave rise to the long-term heavy rainfall.