During 19-21 September of 2001, a rare heavy rain event was suddenly appeared and accompanied with powerful thunderstorm over Mianyang region. Under the condition of insufficient energy is not enough and stable atmospheric stratification after lasting a month with rainy weather. Its forming reasons using meteorology, NWF products, satellite image, and energy condition etc. are analyzed. It's considered that existence and stretch northwards of higher energy tongue with the type Ω in the low layer is a major reason for the heavy rainfall appeared suddenly. It is main energy of the suddenly arising rare torrential rain that releasing of latent heat energy for warm and wet flowing atmosphere climbing along could front face. The feedback speed up the low pressure grow, form the CISK(second unstable condition)mechanism. The article raise a point of view for movement′s energy accumulating, and attempt to explain the reason forming the suddenly arising torrential rain under condition of lower temperature and stable atmosphere.