The potential vorticity distribution at high and low levels and their evolution of vertical structure are discussed based on a case study of the cyclone appeared in Jiang-Huai Valley for the period of 08 July 5-20 July 6.In addition,the relation between the low and the high levels is also studied from the angle of releasing condensation latent heat in the process of the development of the cyclone.The results show that the releasing of condensation latent heat causes the downward transport of the high potential vorticity from high level and leads to the development of low level cyclone, which may be the possible mechanism of the development of Jiang-Huai cyclone.