By using the equation of the kinetic energy in the wavenumberfrequency domain together with the ECMWF/WMO data, the mechanism for the oscillation of the kinetic energy of the mean zonal flow and ultra-long waves at the 200 hPa level over the tropics in the period of 64 days from June 11 to August 13, 1983 is discussed.The oscillation of kinetic energy with a period around 20 days is very obvious. The kinetic energy of the mean zonal flow and waves of wavenumbers 2-7 are all of this oscillation. The kinetic energy of the wave of wavenumber 1 seems to oscillate with a 40-day period. Besides, for waves of wavenumbers 1, 4 and 7, the period around a week of the kinetic energy is also clear.The important reason for the 20-day oscillation of the mean zonal flow is the interaction between the mean flow and waves. However, the reason why the kinetic energy of the waves oscillates is somewhat complex. The conversion of the energy available, the work done by the boundary pressure and the nonlinear interaction between waves are all important.There exists a relation between high and low frequency oscillations of kinetic energy. When the low frequency oscillation gets to its peaks, the amplitude of the high frequency oscillation becomes maximum. When the low frequency oscillation gets to its valleys, the amplitude of the high frequency oscillalion becomes minimum.