In this paper, diurnal rainfall variations in the second rainy season in the coastal and mixed topographical regions of Southeast China (Fujian) were analyzed. The roles of topographical thermal circulations and land-sea breezes in the diurnal variations in thermal convective rainfall were investigated. The impacts of environmental temperature, moisture, and vertical wind profile on the time and intensity of diurnal peaks of convective rainfall were examined using hourly surface data from the automated weather station of Fujian province for the periods July to September of 2009-2017. With radio-soundings at Xiamen station for the periods July to September of 2015-2017 and K-mean clustering methodology, idealized numerical simulations were conducted using a mesoscale numerical model (WRF3.9.1.1). Results were as follows: diurnal rainfall variations for the second rainy season in the study regions of Southeast China (Fujian) were impacted and regulated by topographical thermal circulation and land-sea breezes. The topographical thermal circulation due to daytime heating initiated convective rainfall bands. The bands were organized and peaked during the afternoon sea breezes, then gradually weakened with the weakening topographical thermal circulation and sea breeze. Idealized numerical experiments further proved the role of topographical thermal circulation and sea breezes in the convective rainfall initiation and organization. Environmental temperature, moisture, and vertical wind profiles exerted an important impact on the diurnal peak intensity and time of occurrence of the convective rainfall. The atmospheric lifting condensation level, precipitable water, convective instability, and differential distribution of moisture at the mid-lower levels impacted the diurnal peak intensity of the convective rainfall and changed its time of occurrence through altering its time of initiation. Differences in low-level wind intensity and direction and vertical shear at the mid-lower levels impacted the initiation, organization, and movement of the topographical convection, and further impacted its diurnal peak intensity and time of occurrence.