Abstract: A new generation mechanism of three-band chorus waves in the Earth’s magnetosphere was proposed with Van Allen Probes data, where two obvious power gaps can be observed in the spectrograms. The satellite observations reveal that there usually exist two separated electron beams in the direction opposite wave propagation along with three-band chorus waves. The linear theoretical results confirm that the existence of electron beams can indeed cause two minima of growth rates of chorus waves propagating in the opposite direction of electron beams. Therefore, the two power gaps should be formed due to the severe damping of chorus waves at gap frequencies resulting from the two electron beams.This study not only gives a new mechanism to explain the formation of three-band chorus event, but also provides new insights into understanding the well-known power gap at half electron gyrofrequency.