Abstract: A diagnostic study is presented with respect to budget equations of rotational and divergent components of kinetic energy as well as total potential energy in an open system for a case of strong tropical storm 8014 (Ruth) which occurred and developed in the South China sea. It is shown that for the tropical cyclone of interest, geostrophic effect is the main physical mechanism of conversion from divergent to rotational kinetic energy; diabatic heating supplies most of its energy for development and a large part of the total potential energy produced by it is consumed in subgrid dissipation and lateral boundary outward flux, leaving the remainder to be converted to kinetic energy of the divergent component; two of the functions of conversion [C(P,K_x] and C(K_x,K_ψ) are in good agreement in the distribution of space and time; there is significant exchange of energy between the storm and environment, displayed by the output of total potential energy and rotational kinetic energy at the upper level against the input of divergent kinetic energy at the lower level; the work done by the divergent component acts as major generator of kinetic energy and that by the rotational component as the consumer, in the total budget of energy.