This paper simulated a heavy rainfall case caused by a Jiang-Huai cyclone which occurred in southern Anhui Province from 1200 UTC July 4 to 0600 UTC July 5,2014.The simulated results on the warm front are consistent with the observations,and are used to investigate multiple convective rain belts' structure and the causes for their appearance near the warm front.The results indicate that the single convections in both the simulated and observed situations are all under 300 hPa,and continued for around 2-3 hours,with maximum lengths of 100 km and widths of 10 km.Between bands,their spacings are about 50-100 km,and all of them are tilting east-southern with height.All of the single convective bands have several common circulation characteristics:on the east of the convective region there is a corresponding warm rising air distributed from its low-level to high-level;the upper western convective zones are controlled by cold areas;and close to the ground,there is a similar cold pool,while the updraft south of the convective belts is favored by the convections' back building and forming belted structures.On the single convective rain belts,there is an area of 0 < Ri < 1 at 800 hPa and an area of I_EPV<0.In stable stratification,the wind field is distributed in a roller shape,with the structure characteristics of the existence of gravitational waves.There is convective instability at 600-700 hPa and an area of I_EPV>0.It can be concluded that warm rising air under conditional symmetric unstable conditions around 800 hPa triggered conditional symmetric instability and gravity waves,along with convective instability above 700 hPa.This implies that the main cause of the single convective rain belts' formation is symmetric-convective instability.