Abstract: A brief torrential rain event with a broken-recorder 1h precipitation of 90.8 mm and especially 82 mm received at Tailai in the latter half hour reaching the level in a 100 year return period struck the middle-west of Northeast China on 10 August 2006.This paper is focused on synoptic background, mesoscale environment and triggering mechanism of the mesoscale convective systems (MCSs), by using the conventional and intensive observations from an automatic meteorological stations, satellite source and weather stations. The study of development process of MCS from meso-γto a meso-α convective complex (MCC) by analyzing IR cloud maps and high-resolution visible images ,shows that the MβCS associated to the total of six cities and towns each with >33 mm precipitation in 30 min (heavy rain MCS span divided into two stages, i.e., before and after MCC maturity) is characterized by the fact that at first stage, MβCS moves mainly towards the east, resulting in the mergence into a MCC, and at the second stage, MβCS occurs at the southwestern periphery of the MCC, where the brief strongest rainstorm happens. As shown on the still higher-resolution visible image there are two lines of cumuli and at their meeting location MβCS experiences so vigorous development that torrential rains take place. Study of the causes of MCS intensification and rainstorm shows that( 1) just before the heavy rains occur, there are high temperature, considerably wetness and unsteady convective stratification in the air over the rainstorm-hit area, in addition to significant increase in convectively efficient potential energy, condensation height and decrease in the height of free convection – all these favor the happening of the heavy rain,(2) the mergence of meso-βcloud clusters leads to the MCS such fast growth as to produce rainstorm,( 3) the two lines of cumuli in the north and west correspond, separately, to two convergences on the surface wind field, with stronger convergence at their meeting point responsible for such intense development of meso-βcloud cluster as to bring about torrential rains. Analysis of causes as to why MCS propagates to the southwest of MCC indicates the propagation direction depends on the direction and velocity of the two convergences. Finally, the appearance of northerly flow and the metabolism from old into new clouds act as a key factor for triggering the rainstorm. These results can be a useful reference for foreshadowing short-time torrential rains.