A Meiyu front torrential rain event occurred over the middle and lower reaches of the Yangtze River on 26-28 June 2015.The organization modes and initiation conditions of Mesoscale Convective Systems(MCS) are analyzed by using the conventional meteorological observation data,the dense surface automatic meteorological stations data,Doppler weather radar data and the simulated data from WRF(Weather Research and Forecast) model.The results are as follows:(1)many types of organization modes are found in the development of the linear MCS.Constantly back building of the east-west oriented rainbands and subsequent echo-training of convective cells are the main development modes of the MCS in the initial stage.The convection cells develop in the direction of NE-SW and form several short parallel convective rainbands in a nearly NE-SW direction in the mature stage.Two scales of convective organization modes are found:one is northeastward echo-training of new convective cells along individual rainbands,and the other is eastward band-training of the NE-SW oriented short rainbands along the linear MCS.(2)The continuous enhancement of low-level jet provides the conditional instability and convective available potential energy for the generation and development of convections.The southerly warm and moist air is lifted to the level of free convection,releasing the unstable energy and triggering convection at the convergence of wind speed.(3)The consistent interactions between the cold outflow of convective cells and the low-level southwest warm and moist air are favorable for the generation and development of new cells,maintaining the rainbands.