Using conventional meteorological observations, regional dense automatic weather station data, NCEP/NCAR 1°× 1° reanalysis data with 6 h interval and Doppler radar data, this paper analyzed environmental conditions, echo characteristics and causes of three severe convective weather processes of southwest flow type in Henan in August 2013. The results show that: (1) Three severe convections all occur in the interactions between midlatitude trough and subtropical high. Under the effect of southwest flows from low to high levels, there are temperature and dewpoint gradients and convergence line on the surface. The dynamic effect of highlevel trough and the trigger of convergence line on the surface lead to the convective weather. (2) There are unstable stratifications which are warm and wet in high level and dry and cold in low level. Strong vertical wind shear in the lower troposphere, and the large CAPE and lifting conditions are conducive to occurrence and development of squall lines and super cells. The conditions for the three events are the best on 1 August, better on 11 August and the worst on 7 August. There is a relationship between structure of convective system and the strength of convection forming condition. (3) Monitoring data from radar and dense automatic weather station show that in the initial stage multicell echo structures are dominant, and then gradually strengthen and form multicell echo groups. In the mature stage, they all develop into linear convective systems and even form organized squall lines. The interaction of updraft and downdraft is the main cause for the development and maintenance of squall lines. (4) The deep warm and moist stratification in the lower and the dry layer above that is important to the forecast of severe convective weather in summer in Henan. The indexes such as CAPE, Δθse(850-500), ΔT850-500, K index, and vertical wind shear in the lower troposphere have good indication for southwest flow super cell, squall lines and other severe storms to develop and form.