Based on the local eddy kinetic and available potential energy functions, detailed diagnosis of daily energy evolution is conducted to reveal the relative contribution of several inertial processes such as baroclinic, barotropic processes and eddy ageostrophic geopotential flux, etc., to the maintenance of storm track. It is shown that the "downstream development effect" caused by eddy ageostrophic geopotential fluxes serves as an important factor for the maintenance of track in a preferred region.