In this paper, it is analyzed the diurnal rainfall variations of second rainy-season in the coastal and complicated topographical regions of Southeast China (Fujian), investigated the roles of topographical thermal circulations and land-sea breezes in the formation of diurnal variation of thermal convective rainfall and revealed the impacts of environmental temperature, moisture and vertical wind profiles upon the diurnal peak intensity and occurrence time of thermal convective rainfall with hourly surface data observed by automatic weather station of Fujian province in the periods from July to September of 2009-2017, radio-soundings at Xiamen station in the periods from July to September of 2015-2017, K-mean clustering methodology and idealized numerical simulations with mesoscale numerical model (WRF18.104.22.168). The results show as follows. The diurnal rainfall variation of second rainy-season in the coastal and complicated topographical regions of Southeast China (Fujian) are impacted and modulated by topographical thermal circulation and land-sea breeze. The topographical thermal circulation due to day-time radiation heating has an ability to initiate convective rainfall band. The convective rainfall band is organized and comes to its top under the impacts of sea breeze in the afternoon, then gradually reduces with the weakening of topographical thermal circulation and sea breeze. Idealized numerical experiments further prove the role of topographical thermal circulation in the convective rainfall initiation and that of sea breeze in the convective rainfall organization. The environmental temperature, moisture and vertical wind profiles exert an important impact on the diurnal peak intensity and occurrence time of thermal convective rainfall. The atmospheric lifting condensation level, precipitable water, convective instability and the differential distribution of moisture in the mid-lower level can impact the diurnal peak intensity of thermal convective rainfall and change its occurrence time through altering its initiation time. The differences in low-level wind intensity and direction and vertical shear of mid-lower winds of environmental wind profiles can impact the initiation, organization and movement of topographical thermal convection, and further impact its diurnal peak intensity and occurrence time.