Based on hourly observational datasets from 84 national automatic weather stations and 6 h ERA-Interim reanalysis data at Sichuan Basin between May and September during 2007-2017, this study investigates the ambient conditions, such as thermodynamic variables, water vapor and vertical wind shear, and contrasts characteristics of convective parameters of the occurrence and development of flash-rain under different intensities. Compared with ordinary short-time severe precipitation, extreme flash-rain has relatively higher lifting condensation level (LCL), higher level of free convection (LFC) and higher equilibrium level (EL), which can be used to effectively distinguish extreme and ordinary flash-rain. About 75% of extreme and ordinary flash-rain events occur in the ambient background with EL higher than 258.6 hPa and 658.2 hPa, respectively. The values of convective available potential energy (CAPE) and convective inhibition (CIN) are larger in extreme flash-rain events. About 50% of extreme and ordinary flash-rain occurrence needs CAPE values greater than 792.5 J·kg-1 and 451.9 J·kg-1, respectively. The bigger difference of potential pseudo-equivalent temperature between 850 hPa and 500 hPa (θse850-θse500) is better for extreme flash-rain, and 10℃ can be the threshold value to judge extreme and ordinary flash-rain. The precipitable water (PW) value of about 50% of all flash-rain events is greater than 58 mm. The difference of PW between extreme and ordinary flash-rain is not obvious, but the vertical distribution characteristic of a dry upper level associated with a wet low level of extreme flash-rain is significant. The vertical wind shear can not act as a potential predicator to distinguish flash-rain under different intensities in Sichuan Basin, and the ascending motion can not either.