Based on diagnostic analyses of potential vorticity (PV) and backward trajectory, development of a strong cyclone during the Meiyu period of 2003 is investigated. Results show that diabatic heating is a key in the growth of low-level cyclone at the early stage, and subsequently, due to strengthening of the upper-level horizontal advection, the cyclone rapidly develops as the vertical coupling between the upper- and low-level PV maxima takes place through the vertical advection. In terms of major contributions from different levels to PV column's formation, the diabatic heating is in the low troposphere, the vertical advection in the middle troposphere and the horizontal advection in the upper troposphere. During the rapid development period, the low-level vortex is primarily formed from the southwest warm-wet flow, and the upper-level one is from the downward northwest stratospheric cold-dry flow along the jet axis and upward southwestern low-level tropospheric warm-wet flow which approaches to the northeastern part of the cyclone. The interaction between cold and warm flows mainly occurs over 600 hPa level, and the vertical motion associated with condensational latent heat, through the vertical advection, plays an important role in the interaction between the cold and warm flows and in the vertical coupling between the upper- and low-level PV maxima.