The evolution of mesoscale eddies is crucial for understanding the ocean energy cascade. Using global reanalysis sea surface velocity data and mesoscale eddy trajectory product tracked by satellite altimeters, this study aims to reveal the coherence of eddy kinetic energy (EKE) variation to the low-frequency ageostrophic energy during the eddy life span. The variation of EKE through the eddy life is highly coherent to that of the 7d low-passed ageostrophic kinetic energy with a correlation coefficient of -0.95. The low-frequency ageostrophic motions supply 43% of the EKE variation at the growing stage of mesoscale eddies, and absorb 45% at the decaying stage, comparing to 7% and 9% of contributions for submesoscale motions. The evolution rate of the EKE during the eddy life span is consistent with the barotropic conversion rate of the low-frequency motions, further confirming the dominant role of the low-frequency motions in the eddy growing and decaying.