Abstract: The ECMWF grid data from November 1984 to Feburary 1985 and from May to August 1985 were used to calculate the distributions of the kinetic energy and barotropic kinetic energy of low-frequency (40-50 day) fluctuations and the interaction of time-averaged flows with low-frequency fluctuations in the northern Hemisphere.The results reveal that the high value areas of the kinetic energy of low-frequency fluctuations in the upper level are closely related with the zonal asymmetry and the meridional shear of westerlies.The time-averaged flow transfers kinetic energy to low-frequency fluctuations in the exit area of westerlies, especially in the exit area of westerlies over the east Asia, where the law-frequency barotropic kinetic energy is the largest.Thelow-frequency barotropic kinetic energy in the Asian monsoon area is smaller.It is suggested that the ratio of low-frequency barotropic energy to low-frequency kinetic energy can be served as a " structure parameter" of low-frequency fluctuations and the isopleth of 0.5 of the parameter as a demarcation line between tropical low-frequency oscillations and mid-high latitude low-frequency oscillations.