Abstract: Dispersion relation and energy equation of tropical vortex are obtained by using WKB method. The condition and source of energy for the development of instability of tropical vortex were discussed. The detailed analysis show that non-homogeneities in the radial velocity of the base field is a important factor, and it gives rise to strong inflow in low layer of tropical vortex. The mechanism of nondeveloping depression being transfered into developing systems is explained by the use of theoretical results.