Abstract: The warm-sector rainstorm is one of the major weather disasters in southern Henan in flood season, and it is complex in developing mechanisms and has obvious regional characteristics. Based on the station observation data, the sounding data and the European central atmospheric reanalysis datasets (ERA-5), the synoptic characteristics of warm-sector rainstorms over southern Henan are investigated by the mesoscale analysis and induction for the weather condition of warm-sector rainstorms from May to September in flood season during 2011—2020. Accordingly, the thresholds of environmental condition parameters related to warm-sector rainstorms with different circulation patterns are extracted by percentile method. The results are shown as follows: (1) The warm-sector rainstorm is an important component of the flood season in southern Henan, and 36 warm-sector rainstorm days are identified from May to September during 2011—2020, accounting for about 53% of the total heavy precipitation days. (2) Warm-sector heavy rainfalls are categorized into four types of synoptic conceptual models in southern Henan, including warm shear line, shear-subtropical high, strong southwest jet and cold front types, in which the shear-subtropical high type occurs most frequently, and the cold front type is the least. The frequency distribution of warm-sector rainstorms in four types is closely related to the location of their major weather systems. (3) Conditions with abundant water vapor in the whole layer, relatively strong deep vertical wind shear, moderate instability energy of atmosphere and low-level southerly jet are conducive to generate warm-sector rainstorms in southern Henan. The specific humidity at 850 hPa, the depression of the dew point at 500 hPa, and the maximum southerly wind speed at the lower level can be applied to indicate the forecast of warm-sector rainstorms over southern Henan.